Radiative Rate Constants Research Articles

Are Triphenylamine-Functionalized or Carbazole-Functionalized Iridium Complexes the More Effective Phosphorescent Materials? A Theoretical Perspective

The ground and excited states, charge injection/transport, and phosphorescence properties of eleven carbazole- and triphenylamine-functionalized Ir(III) complexes were investigated by using the DFT method. By analyzing the spin-orbit coupling (SOC) matrix elements, radiative decay rate constants k(r), and the electronic structures and energies at the S₀(opt) and T₁(opt) states, it was possible to rationalize the order of the experimental phosphorescence quantum yields of a series of Ir(III) complexes and to predict that [Ir(Nph-2-Cz-tz)3] has a higher phosphorescence quantum yield than [Ir(TPA-tz)3] (TPA=triphenylamine, tz=thiazolyl, Cz=carbazole, Nph=N-phenyl). Carbazole-functionalized Ir(III) complexes were shown to be efficient phosphorescent materials that have not only fast but also balanced electron/hole-transport performance as well as high phosphorescence quantum yields. The phosphorescence emission spectra can be modulated by modifying or replacing a pyridyl substituent.

Theoretical Studies on Phosphorescent Materials: The Conjugation-Extended PtII Complexes

A theoretical study on the PtII complex A based on a dimesitylboron (BMes2)-functionalized [Pt(C^N)(acac)] (C^N = 2-phenyl-pyridyl, acac = acetylaceton) complex, as well as three conjugation-extended analogues of the methylimidazole (C*) ligand BMes2-[Pt(C^C*)(acac)] complexes B–D is performed. Their theoretical geometries, electronic structures, emission properties, and the radiative decay rate constants (kr) were also investigated. The energy differences between the two highest occupied orbitals with dominant Pt d-orbital components (Δddocc) of D both at the ground and excited states are the smallest of all. Compared with B, the charge transfer in D possesses a marked trend towards the extended conjugated group, while C changed inconspicuously. The lowest-lying absorptions and the phosphorescence of them can be described as a mixed metal-to-ligand charge transfer (MLCT)/intra-ligand π→π* charge transfer (ILCT) and 3MLCT/3ILCT, respectively. The variation of charge transfer properties induced by extended conjugation and the radiative decay rate constants (kr) calculated revealed that D is a more efficient blue phosphorescence material with a 497 nm emission transition.

A DFT/TDDFT study on the effect of CN substitution on color tuning and phosphorescence efficiency of a series of Ir(iii) complexes with phosphine-silanolate ligands

A DFT/TDDFT investigation was performed on the electronic structures, absorption and emission spectra, as well as the phosphorescence efficiency of [(ppy)2Ir(P^SiO)] (1) and the derivatives (1a, 1b, 1c and 1d) with CN-substitution at different positions in ppy ligands, as well as [(dfppy)2Ir(P^SiO)] (2) [where ppy = 2-phenylpyridine, dfppy = 2-(2,4-difluorophenyl)pyridine and (P^SiO) is an organosilanolate ancillary chelate]. The calculated results reveal that the introduction of CN leads to a significant red shift for 1a-1d in absorption spectra compared with that of 1. Moreover, the CN substitution at different positions on C^N ligands may be an efficient approach towards tuning emitting color. 1b, 1c, and 1d lead to a blue shift of emission spectra compared with 1, while an obvious red shift is observed for 1a. The high quantum yield of 1 (0.90) compared to 2 (0.59) is explained based on the S1-Tn splitting energies and energy gap between (3)MLCT/π-π* and (3)MC d-d states, and the evaluation of the radiative and nonradiative rate constants for all the complexes is also studied. The designed complexes 1a, 1c and 1d are expected to be potential phosphorescence emitters in OLEDs with high quantum efficiency.

Sensitive Time-Correlated Single Photon Counting Enables Efficient Singlet Oxygen Detection

Single photon counting based data acquisition has proven to yield a major sensitivity increase in the optical evaluation of pharmaceuticals and biotechnology products. We will show for the first time that a state of the art time-correlated single photon counting (TCSPC) based fluorescence lifetime spectrometer is able to quantify singlet oxygen generation and to characterize the singlet oxygen phosphorescence decay. This makes TCSPC based fluorescence lifetime spectrometers a valuable tool for studying photosensitizers widely used for example in photodynamic therapy (PDT). The detection of the faint singlet oxygen phosphorescence signal has been made possible by using a special burst mode for the pulsed laser excitation and a new generation of TCSPC electronics with a significantly reduced dead-time which enables efficient multi-stop photon detection.Thanks to a recently developed integrating sphere add-on we are also able to measure fluorescence quantum yields with the same instrument. Leveraging the possibility to measure fluorescence lifetime in conjunction with quantum yield, we performed a systematic investigation of the relation between reduced fluorescence emission and different contributions of dynamic and static quenching processes.Furthermore, since quenching normally does not affect the radiative rate constant, this combined set-up allows to verify the accuracy of the extracted lifetime. Especially for very short lifetimes in the range of the instrument response function the presented method allows to assess whether the proper fluorescence lifetime was extracted.

Non-classical donor–acceptor–donor chromophores. A strategy for high two-photon brightness

Rod-like π-expanded bis-imidazo[1,2-a]pyridines were designed and synthesized. Strategic placement of a central electron-poor unit at position C3 of the imidazo[1,2-a]pyridine core resulted in donor–acceptor–donor quadrupolar systems. We further demonstrated the applicability of the Ortoleva–King–Chichibabin tandem process in the preparation of complex imidazo[1,2-a]pyridines. The monomer model heterocycles, differing by substitution at C2 and C3, displayed luminescence properties marginally dependent on the substitution pattern and conjugation extension. The highest luminescence quantum yield (ϕflca. 0.2) is shown by the model compounds without a substitution at C3, whereas lower values are measured for the others. Quantum yields in toluene for the quadrupolar systems varied from ϕfl = 0.7 to ϕfl > 0.9. Apart from bis-imidazo[1,2-a]pyridine possessing two methyl groups, with ϕfl = 0.69 in toluene and 0.35 in dichloromethane, the luminescence properties were only slightly affected by the change in the solvent. Most of the fluorescence lifetimes ranged from 1–2 ns and the radiative rate constants reached values of ca. 6 × 108 s−1. Intense phosphorescence spectra at 77 K with lifetimes in the second range (τ = 0.6–1.3 s) are recorded for the monomers, whereas for the bis-imidazo[1,2-a]pyridines, phosphorescence spectra are detected only after enhancing intersystem crossing by heavy atoms. Non-linear optical properties of bis-imidazo[1,2-a]pyridines revealed two-photon absorption cross-sections (σ2) typically of the order of 500–800 GM. The remarkable fluorescence quantum yield, in combination with high a two-photon absorption cross-section, generated a two-photon brightness of ∼500 to 750 GM units within the biological window (700–800 nm).

Tuning the electronic properties and quantum efficiency of blue Ir(iii) carbene complexes via different azole-pyridine-based N^N′ ligands

The electronic structures and photophysical properties of a series of heteroleptic Ir(III) carbene complexes (fpmi)3−xIr(N^N′)x (x = 1, 2) [fpmi = 1-(4-fluorophenyl)-3-methylimidazolin-2-ylidene-C,C2′, N^N′ = 2-(1H-pyrrol-2-yl)pyridinato (1a(x = 1)/1a′(x = 2)); 2-(1H-pyrazol-5-yl)pyridinato (2a/2a′); 2-(1H-1,2,4-triazol-5-yl)pyridinato (3a/3a′); 2-(1H-tetrazol-5-yl)pyridinato (4a/4a′)] with different azole-pyridine-based N^N′ ligands as ancillary and main chelate, respectively, are investigated using the density functional method. It is found that, with the systematic addition of N substitution in N^N′ ligands, the HOMO–LUMO energy gap follows a decreasing order from 1a to 4a with ancillary N^N′ ligands, but increasing order for 1a′–4a′ with main N^N′ ligands. Besides, the emission spectra are blue shifted with the addition of N substitution and the emissive state of all of the studied complexes is predominantly controlled by the N^N′ ligand, regardless of being ancillary or the main chelate. Furthermore, the evaluation of the radiative (kr) and nonradiative (knr) rate constants for all of the complexes are also investigated based on the calculated results. It's found that the quantum yield (ΦPL) shows an apparent dependence on the selection of different N^N′ ligands and the N^N′ ligands as main chelate present a small kr and large knr, which is not beneficial for efficient materials. While the pyrazole- and 1,2,4-triazole-pyridine-based N^N′ ligands as ancillary chelate is believed to be more favorable for highly efficient phosphorescence emitters in OLEDs.

Investigation of energy transfer from pyrromethene dye to cresyl violet 670 in ethanol

In this paper, radiative and nonradiative energy transfer from laser dye pyrromethene 567 (PM567) and pyrromethene 580 (PM580) as donors to cresyl violet 670 (CV670) as acceptor in ethanol are investigated by using the steady-state emission measurement and the second harmonic generation (532 nm, ∼ 13 ns) of a Q-switched Nd:YAG laser as the pumping source. The fluorescence intensity of the acceptor is improved due to the introduction of the donors, and the largest enhancement is obtained to be 128% in the PM567:CV670 dye mixture system. Energy transfer parameters, including the radiative and nonradiative energy transfer rate constants (KR and KNR), critical distance (R0), and half quenching concentration ([A]1/2) are investigated using the Stern—Volmer plots, and the acceptor concentration dependencies of radiative and nonradiative transfer efficiencies are also obtained. The values of KR for PM567:CV670 and PM580:CV670 systems are 2032.0×109 L·mol−1·s−1 and 2790.4×109 L·mol−1·s−1, respectively, and the values of corresponding KNR are 3.3 × 109 L·mol−1·s−1 and 4.2×109 L·mol−1·s−1, respectively. The results indicate that the dominant mechanism responsible for the energy transfer in the dye mixture systems is of the radiative type.

Photophysics of Glycosylated Derivatives of a Chlorin, Isobacteriochlorin and Bacteriochlorin for Photodynamic Theragnostics: Discovery of a Two‐photon‐absorbing Photosensitizer

The photophysical properties of a chlorin, isobacteriochlorin and bacteriochlorin built on a core tetrapentafluorophenylporphyrin (TPPF20 ) and the nonhydrolyzable para thioglycosylated conjugates of these chromophores are presented. The photophysical characterization of these compounds was done in three different solvents to correlate with different environments in cells and tissues. Compared with TPPF20 other dyes have greater absorption in the red region of the visible spectrum and greater fluorescence quantum yields. The excited state lifetimes are from 3 to 11 ns. The radiative and nonradiative rate constants for deactivation of the excited state were estimated from the fluorescence quantum yield and excited state lifetime. The data indicate that the bacteriochlorin has strong absorption bands near 730 nm and efficiently enters the triplet manifold. The isobacteriochlorin has a 40-70% fluorescence quantum yield depending on solvent, so it may be a good fluorescent tag. The isobacteriochlorins also display enhanced two-photon absorption, thereby allowing the use of 860 nm light to excite the compound. While the two-photon cross section of 25 GM units is not large, excitation of low chromophore concentrations can induce apoptosis. The glycosylated compounds accumulate in cancer cells and a head and neck squamous carcinoma xenograft tumor model in mice. These compounds are robust to photobleaching.

Temperature Effect on Radiative Lifetimes: The Case of Singlet Oxygen in Liquid Solvents

A change in solvent can have an appreciable effect on the rate constant for the O2(a(1)Δg) → O2(X(3)Σg(-)) radiative transition at ~1275 nm. The data thus obtained have played an important role in understanding mechanisms by which environment-dependent perturbations can influence forbidden electronic transitions. We now report that the rate constant for O2(a(1)Δg) radiative deactivation, kr, also responds to changes in temperature. This result can have practical ramifications in experiments that use O2(a(1)Δg) phosphorescence to quantify yields of photosensitized O2(a(1)Δg) production. From a fundamental perspective, this result is significant, partly because there is little precedence for temperature-dependent changes in radiative rate constants. The data also require a re-evaluation of the current model by which oxygen is perturbed by solvent. Specifically, the evidence indicates that it is not appropriate to evaluate the interaction as a 1:1 complex between a given solvent molecule M and oxygen. Rather, one must consider an ensemble of solvent molecules surrounding oxygen.

Triphenylamine–Benzimidazole Derivatives: Synthesis, Excited-State Characterization, and DFT Studies

The synthesis and comprehensive characterization of the excited states of four novel triphenylamine-benzimidazole derivatives has been undertaken in solution (ethanol and methylcyclohexane) at room temperature. This includes the determination of the absorption, fluorescence, and triplet-triplet absorption spectra, together with quantum yields of fluorescence, internal conversion, intersystem crossing, and singlet oxygen. From the overall data the radiative and radiationless rate constants could be obtained, and it is shown that the compounds are highly emissive with the radiative decay dominating, with more than 70% of the quanta loss through this deactivation channel. The basic structure of the triphenylamine-benzimidazole derivatives (1a) was modified at position 5 of the heterocyclic moiety with electron-donating (OH (1b), OCH3 (1c)) or electron-withdrawing groups (CN, (1d)). It was found that the photophysical properties remain basically unchanged with the different substitutions, although a marked Stokes shift was observed with 1d. The presence and nature of a charge-transfer transition is discussed with the help of theoretical (DFT and TDFT) data. All compounds displayed exceptionally high thermal stability (between 399 and 454 °C) as seen by thermogravimetric analysis.

Photophysical Properties of Cyclometalated Pt(II) Complexes: Counterintuitive Blue Shift in Emission with an Expanded Ligand π System

A detailed examination was performed on photophysical properties of phosphorescent cyclometalated (C(^)N)Pt(O(^)O) complexes (ppy)Pt(dpm) (1), (ppy)Pt(acac) (1'), and (bzq)Pt(dpm) (2) and newly synthesized (dbq)Pt(dpm) (3) (C(^)N = 2-phenylpyridine (ppy), benzo[h]quinoline (bzq), dibenzo[f,h]quinoline (dbq); O(^)O = dipivolylmethanoate (dpm), acetylacetonate (acac)). Compounds 1, 1', 2, and 3 were further characterized by single crystal X-ray diffraction. Structural changes brought about by cyclometalation were determined by comparison with X-ray data from model C(^)N ligand precursors. The compounds emit from metal-perturbed, ligand-centered triplet states (E(0-0) = 479 nm, 1; E(0-0) = 495 nm, 2; E(0-0) = 470 nm, 3) with disparate radiative rate constants (kr = 1.4 × 10(5) s(-1), 1; kr = 0.10 × 10(5) s(-1), 2; kr = 2.6 × 10(5) s(-1), 3). Zero-field splittings of the triplet states (ΔE(III-I) = 11.5 cm(-1), 1'; ΔE(III-I) < 2 cm(-1), 2; ΔE(III-I) = 46.5 cm(-1), 3) were determined using high resolution spectra recorded in Shpol'skii matrices. The fact that the E0-0 energies do not correspond to the extent of π-conjugation in the aromatic C(^)N ligand is rationalized on the basis of structural distortions that occur upon cyclometalation using data from single crystal X-ray analyses of the complexes and ligand precursors along with the triplet state properties evaluated using theoretical calculations. The wide variation in the radiative rate constants and zero-field splittings is also explained on the basis of how changes in the electronic spin density in the C(^)N ligands in the triplet state alter the spin-orbit coupling in the complexes.

Effect of Ligand Polarization on Asymmetric Structural Formation for Strongly Luminescent Lanthanide Complexes

AbstractThe effect of ligand polarization on the formation of strongly luminescent lanthanide complexes with asymmetric structures is described for the first time. The lanthanide complexes are composed of EuIII ions, three hexafluoroacetylacetonate (hfa) ligands, and two monodentate phosphine oxide ligands, namely, triphenylphosphine oxide (TPPO), p‐tolyldiphenylphosphine oxide (TPPO‐Me), tri‐p‐tolylphosphine oxide (TPPO‐3Me) or o‐phenoxyphenyldiphenylphosphine oxide (TPPO‐OPh). The luminescence properties of the EuIII complexes are characterized by their emission quantum yields, emission lifetimes, and their radiative (kr) and nonradiative (knr) rate constants. The EuIII complex with TPPO‐OPh ligands offers a markedly high emission quantum yield (72 % in [D6]acetone, 85 % in the solid state) owing to enhancement of the electric dipole transition and suppression of vibrational relaxation, which are directly related to kr and knr. The coordination geometries of the EuIII complexes are categorized by shape‐measure calculations. The EuIII complexes exhibit characteristic square‐antiprismatic or trigonal‐dodecahedral structures, depending on the ligand polarization. Strongly luminescent Eu(hfa)3(TPPO‐OPh)2 has an asymmetric dodecahedron structure. The formation of distorted dodecahedral structures with low vibrational frequencies for the enhancement of luminescence is elucidated in terms of the ligand polarization of the monodentate phosphine oxide ligands in the EuIII complexes.

Computational Studies on the Excited States of Luminescent Platinum(II) Alkynyl Systems of Tridentate Pincer Ligands in Radiative and Nonradiative Processes

Platinum(II) alkynyl complexes of various tridentate pincer ligands, [Pt(trpy)(C≡CR)](+) (trpy = 2,2':6',2″-terpyridine), [Pt(R'-bzimpy)(C≡CR)](+) (R'-bzimpy = 2,6-bis(N-alkylbenzimidazol-2'-yl)pyridine and R' = alkyl), [Pt(R'-bzimb)(C≡CR)] (R'-bzimb = 1,3-bis(N-alkylbenzimidazol-2'-yl)benzene and R' = C4H9), have been found to possess rich photophysical properties. The emission in dilute solutions of [Pt(trpy)(C≡CR)](+) originated from a triplet alkynyl-to-tridentate pincer ligand-to-ligand charge transfer (LLCT) excited state, with mixing of a platinum-to-tridentate pincer ligand metal-to-ligand charge transfer (MLCT) excited state, while that of [Pt(R'-bzimb)(C≡CR)] originated from a triplet excited state of intraligand (IL) character of the tridentate ligand mixed with a platinum-to-tridentate ligand MLCT character. Interestingly, both emissions were observed in [Pt(R'-bzimpy)(C≡CR)](+) in some cases. In addition, [Pt(R'-bzimb)(C≡CR)] displayed a photoluminescence quantum yield higher than that of [Pt(R'-bzimpy)(C≡CR)](+). Computational studies have been performed on the representative complexes [Pt(trpy)(C≡CPh)](+) (1), [Pt(R'-bzimpy)(C≡CPh)](+) (2), and [Pt(R'-bzimb)(C≡CPh)] (3), where R' = CH3 and Ph = C6H5, to provide an in-depth understanding of the nature of their emissive origin as well as the radiative and nonradiative processes. In particular, the factors governing the ordering of the triplet excited states and radiative decay rate constants of the emissive state ((3)ES) have been examined. The potential energy profiles for the deactivation process from the (3)ES via triplet metal-centered ((3)MC) states have also been explored. This work reveals for the first time the potential energy profiles for the thermal deactivation pathway of square planar platinum(II) complexes.

Highly Luminescent Dinuclear Platinum(II) Complexes Incorporating Bis-Cyclometallating Pyrazine-Based Ligands: A Versatile Approach to Efficient Red Phosphors

A series of luminescent dinuclear platinum(II) complexes incorporating diphenylpyrazine-based bridging ligands (L(n)H2) has been prepared. Both 2,5-diphenylpyrazine (L(2)H2) and 2,3-diphenylpyrazine (L(3)H2) are able to undergo cyclometalation of the two phenyl rings, with each metal ion binding to the two nitrogen atoms of the central heterocycle, giving, after treatment with the anion of dipivaloyl methane (dpm), complexes of formula {Pt(dpm)}2L(n). These compounds are isomers of the analogous complex of 4,6-diphenylpyrimidine (L(1)H2). Related complexes of dibenzo(f,h)quinoxaline (L(4)H2), 2,3-diphenyl-quinoxaline (L(5)H2), and dibenzo[3,2-a:2',3'-c]phenazine (L(6)H2) have also been prepared, allowing the effects of strapping together the phenyl rings (L(4)H2 and L(6)H2) and/or extension of the conjugation from pyrazine to quinoxaline (L(5)H2 and L(6)H2) to be investigated. In all cases, the corresponding mononuclear complexes, Pt(dpm)L(n)H, have been isolated too. All 12 complexes are phosphorescent in solution at ambient temperature. Emission spectra of the dinuclear complexes are consistently red shifted compared to their mononuclear analogues, as are the lowest energy absorption bands. Electrochemical data and TD-DFT calculations suggest that this effect arises primarily from stabilization of the LUMO. Introduction of the second metal ion also has the effect of substantially increasing the molar absorptivity and, in most cases, the radiative rate constants. Meanwhile, extension of conjugation in the heterocycle of L(5)H2 and L(6)H2 and planarization of the aromatic system favored by interannular bond formation in L(4)H2 and L(6)H2 leads to further red shifts of the absorption and emission spectra to wavelengths that are unusually long for cyclometalated platinum(II) complexes. The results may offer a versatile design strategy for tuning and optimizing the optical properties of d-block metal complexes for contemporary applications.

Strongly Blue Luminescent Cationic Iridium(III) Complexes with an Electron‐Rich Ancillary Ligand: Evaluation of Their Optoelectronic and Electrochemiluminescence Properties

AbstractTwo strongly blue luminescent cationic heteroleptic iridium complexes 1b and 2b bearing a 4,4′‐bis(dimethylamino)‐2,2′‐bipyridine (dmabpy) ancillary ligand and either 1‐benzyl‐4‐(2,4‐difluorophenyl)‐1H‐1,2,3‐triazole (dFphtl) or 2‐(2,4‐difluorophenyl)‐5‐methylpyridine (dFMeppyH), respectively, have been synthesized and fully characterized. In comparison with other analogues, the interplay of the triazole unit with the dmabpy unit and methylation of the pyridine ring are discussed with respect to the photophysical, electrochemical, and electrochemiluminescent (ECL) properties of the complexes. The two complexes, 1b and 2b, are blue emitters with λmax = 495 and 494 nm, respectively. The nature of the excited states was established by various photophysical and photochemical experiments as well as DFT calculations. Both complexes emit from a ligand‐centered state, however, the emission of 1b possesses significant charge‐transfer character, which is absent in 2b. The presence of the methyl group on the cyclometalating ligand leads only to a modest increase in the radiative rate constant, kr, but otherwise does not appreciably influence the optoelectronic properties of the complex compared with the non‐methylated analogue. In contrast, the efficacy of the ECL emission when scanning to 2.50 V is strongly influenced by the presence of the methyl group. ECL emission is also enhanced in complexes bearing dmabpy ancillary ligands compared with those containing dtBubpy ligands. The two complexes exhibit similar electrochemical behavior. Incorporation of the dmabpy ligand shifts both the oxidation and reduction cathodically. The combination of the dmabpy and dFphtl groups increases the redox potential difference and thus the HOMO–LUMO gap but the emission is not further blueshifted. Thus, the structural modification of the cyclometalating ligand, although only modestly tuning the emission energy, modulates the nature of the excited state and the efficiency of the ECL process.

Chloride Ion-Pairing with Ru(II) Polypyridyl Compounds in Dichloromethane

Chloride ion-pairing with a series of four dicationic Ru(II) polypyridyl compounds of the general form [Ru(bpy)3-x(deeb)x](PF6)2, where bpy is 2,2'-bipyridine and deeb is 4,4'-diethylester-2,2'-bipyridine, was observed in dichloromethane solution. The heteroleptic compounds [Ru(bpy)2(deeb)](2+) and [Ru(bpy)(deeb)2](2+) were found to be far less sensitive to ligand loss photochemistry than were the homoleptic compounds [Ru(bpy)3](2+) and [Ru(deeb)3](2+) and were thus quantified in most detail. X-ray crystal structure and (1)H NMR analysis showed that, when present, the C-3/C-3' position of bpy was the preferred site for adduct formation with chloride. Ion-pairing was manifest in UV-visible absorption spectral changes observed during titrations with TBACl, where TBA is tetrabutyl ammonium. A modified Benesi-Hildebrand analysis yielded equilibrium constants for ion-pairing that ranged from 13 700 to 64 000 M(-1) and increased with the number of deeb ligands present. A Job plot indicated a 2:1 chloride-to-ruthenium complex ratio in the ion-paired state. The chloride ion was found to decrease both the excited state lifetime and the quantum yield for photoluminescence. Nonlinear Stern-Volmer plots were observed that plateaued at high chloride concentrations. The radiative rate constants decreased and the nonradiative rate constants increased with chloride concentration in a manner consistent with theory for radiative rate constants and the energy gap law. Equilibrium constants for excited state ion-pairing abstracted from such data were found to be significantly larger than that measured for the ground state. Photophysical studies of hydroxide and bromide ion-pairing with [Ru(bpy)2(deeb)](2+) are also reported.

A Comprehensive Strategy to Boost the Quantum Yield of Luminescence of Europium Complexes

Lanthanide luminescence has many important applications in anion sensing, protein recognition, nanosized phosphorescent devices, optoelectronic devices, immunoassays, etc. Luminescent europium complexes, in particular, act as light conversion molecular devices by absorbing ultraviolet (UV) light and by emitting light in the red visible spectral region. The quantum yield of luminescence is defined as the ratio of the number of photons emitted over the number of UV photons absorbed. The higher the quantum yield of luminescence, the higher the sensitivity of the application. Here we advance a conjecture that allows the design of europium complexes with higher values of quantum yields by simply increasing the diversity of good ligands coordinated to the lanthanide ion. Indeed, for the studied cases, the percent boost obtained on the quantum yield proved to be strong: of up to 81%, accompanied by faster radiative rate constants, since the emission becomes less forbidden.

Photophysics of Soret-excited free base tetraphenylporphyrin and its zinc analog in solution

Photophysical properties of free base tetraphenylporphyrin and its zinc analog are investigated in detail in solvents of varying polarity by using steady state and time-resolved techniques. Both the porphyrins are excited at the Soret band to have better signal-to-noise ratio. Also, the fluorescence emission measurements are carried out by using dilute solutions (∼10−7mol/L) of the fluorophores in order to minimize the self-quenching effect. It is observed that the steady state absorption and emission characteristics of the porphyrin molecules are mainly affected by polarizability (via refractive index) rather than polarity (via dielectric constant) of the moderate to highly polar solvents. As the molecules are highly symmetric in the ground state, the associated dipole moments are found to be very low from quantum chemical calculations performed by density functional theory method by using Gaussian 03 package. The dipole moments associated with the first excited singlet state of the porphyrins are computed by applying solvatochromic Stokes’ shift method. To the best of our knowledge, this is the first attempt to calculate the excited state dipole moments of the porphyrins used in the present investigations. Also, fluorescence quantum yield, fluorescence lifetime of the first excited singlet state, radiative and non-radiative rate constants of the porphyrins are reported in solvents of varying polarity.

Investigation of energy transfer between PM567:Rh610 dye mixture in modified poly (methyl methacrylate)

In this paper, solid dye samples were prepared by codoping laser dyes Pyrromethene 567 (PM567) as the energy donor and Rhodamine 610 (Rh610) as the energy acceptor into the ethanol modified poly (methyl methacrylate) matrix (MPMMA) to enhance the properties of the solid dye lasers. The fluorescence intensity of the acceptor was enhanced by up to 9 fold with the introduction of the donor molecules. The laser efficiency of the dye mixture doped samples was improved by up to 8 times relative to that of the samples solely doped with the acceptor, and the highest slope efficiency was obtained as 70.4%. The radiative and nonradiative energy transfer rate constants (KR and KNR) were calculated using the Stern–Volmer plots and the acceptor concentration dependence of the radiative and nonradiative transfer efficiencies were also obtained. The KR was three orders of magnitude higher than the KNR, indicating the dominance of the radiative energy transfer mechanism in the present system. The deviation of the Stern–Volmer plot from the linearity demonstrated that both the dynamic and transient quenching mechanism exist in the present energy transfer system.

Determination of photodissociation and radiative association cross sections from the same time-dependent calculation

We illustrate some of the difficulties that may be encountered when computing photodissociation and radiative association cross sections from the same time-dependent approach based on wavepacket propagation. The total and partial photodissociation cross sections from the 33 vibrational levels of the b 3Σ+ state of HeH+ towards the nine other 3Σ+ and 6 3Π n = 2, 3 higher lying electronic states are calculated, using the autocorrelation method introduced by Heller (1978 J. Chem. Phys. 68 3891) and the method based on the asymptotic behaviour of wavepackets introduced by Balint-Kurti et al (1990 J. Chem. Soc. Faraday Trans. 86 1741). The corresponding radiative association cross sections are extracted from the same calculations, and the photodissociation and radiative association rate constants are determined.