Radiative Decay Constants Research Articles

Photophysical properties of 3MPBA: Evaluation and co-relation between solvatochromism and quantum yield in different solvents

The present paper addresses the photophysical properties of 3-Methoxyphenyl boronic acid (3MPBA) by using solvatochromic shift and quantum chemical methods. The absorption and fluorescence spectra of newly synthesized aryl boronic acid derivative (3MPBA) have been recorded in various solvents of different polarities. The dipole moments were estimated using quantum chemical calculations and Solvatochromic correlations. It is observed that the excited state dipole moments (μe) are greater than the ground state dipole moment (μg) which confirms π→π* transition. And it is also observed that the ground state and excited state dipole moments are observed to be collinear to each other. The changes in dipole moment (Δμ) were also calculated both from solvatochromic shift method and microscopic solvent polarity parameter (ETN) and values are compared. The spectral variations were also analyzed by Kamlet-Taft parameters. It is found that HBA (β) influence is more than HBD (α) for ν̅a and ν̅f in alcohols whereas HBD (α) influence is more than HBA (β) for ν̅a and ν̅f in non alcohols. Further, the relative quantum yield (Φ), radiative and non-radiative decay constants are estimated by single point technique which concludes that the 3MPBA is more radiative in nature and less intersystem crossing and inter conversions are observed in the excited state.

Low amplified spontaneous emission threshold and suppression of electroluminescence efficiency roll-off in layers doped with ter(9,9′-spirobifluorene)

We demonstrate that ter(9,9′-spirobifluorene) (TSBF) doped in a host matrix layer of 4,4′-bis(carbazol-9-yl)biphenyl (CBP) shows a low amplified spontaneous emission (ASE) threshold (Eth = 1.0 μJ cm−2) and suppressed electroluminescence efficiency roll-off at high current densities (no roll-off up to 100 mA cm−2). One origin of the low ASE threshold is that the TSBF-doped CBP layer possesses a very large radiative decay constant (kr = 1.1 × 109 s−1). Singlet–triplet annihilation is almost suppressed in the TSBF-doped CBP layer, which can be ascribed to the small overlap between the emission and triplet absorption of TSBF. Also, the small energy level difference between TSBF and CBP minimizes carrier trapping in TSBF, leading to the suppression of singlet–polaron annihilation. TSBF showed one of the lowest Eth and the most suppressed efficiency roll-off among organic laser dyes investigated in this study and, therefore, is believed to be a promising candidate to realize electrically pumped organic semiconductor laser diodes in the future.

Structural and photophysical properties of alkylamino substituted 2-arylidene and 2,5-diarylidene cyclopentanone dyes

A series of alkylamino substituted 2-arylidene and 2,5-diarylidene cyclopentanone compounds were synthesized. Spectroscopic and photophysical properties of these compounds have been measured in a variety of solvents. Absorption and fluorescence maxima have been correlated with the Empirical Scale of Solvent Polarity (ET(30)). Theoretical TD-DFT spectral calculations and Lippert–Mataga analysis support the internal charge transfer (ICT) nature of the S0 → S1 excitation for these compounds, with higher degrees of ICT depicted for the alkylamino substituted 2,5-diarylidene cyclopentanones. Photophysical properties consisted of measuring the fluorescence quantum yields (Φf) and lifetimes (τf) in a variety of solvents. Radiative and nonradiative decay constants have been determined from the Φf and τf data. Variation with solvent in the nonradiative rate of decay is interpreted in terms of a competition between internal conversion and intersystem crossing. Lastly, two compounds presented have been shown to undergo excited state protonation in glacial acetic acid.

Luminescent Main-Chain Organoborane Polymers: Highly Robust, Electron-Deficient Poly(oligothiophene borane)s via Stille Coupling Polymerization

A series of polymers (PBnT, n = 2–5) with boron atoms incorporated into the conjugated polythiophene main chain have been prepared via Pd-catalyzed coupling of stannylated thienylborane monomers. The polymers exhibit excellent long-term chemical stability to air and moisture and remarkable thermal stability with decomposition temperatures reaching over 300 °C. The high stability is achieved by placing very bulky pendant groups, 2,4,6-tri-tert-butylphenyl (Mes*) and 2,4,6-tris(trifluoromethyl)phenyl (FMes), on boron that prevent attack by nucleophiles. All these polymers display strong absorptions in the visible region and intense fluorescence in both solution and the solid state with quantum yields of up to 38% and fast radiative decay constants (kr) of up to 3.3 × 108 s–1. Density functional theory (DFT) studies on diborylated oligothiophene model compounds suggest that the strong absorption of the polymers results from π–π* transitions on the oligothiophene borane main chain with significant charge tran...

Specific interactions of alcohols and non-alcohols with a biologically active boronic acid derivative: a spectroscopic study.

The photophysical properties of 4-fluoro-2-methoxyphenyl boronic acid (4FMPBA) are characterized using absorption and fluorescence techniques in series of non-alcohols and alcohols. The results are analyzed using different solvent polarity functions and Kamlet and Catalan's multiple regression approaches. The excited state dipole moment and change in dipole moment are calculated using both the solvatochromic shift method and Reichardt's microscopic solvent polarity parameter ETN. The ground state dipole moment is evaluated using quantum chemical calculations. It is found that general solute-solvent and hydrogen bond interactions are operative in this system. A red shift of ~ 9 nm in the emission spectra is observed with an increase in the solvent polarity, which depicts π→π(*) transitions, as well as the possibility of an intramolecular charge transfer (ICT) character in the emitting singlet state of 4FMPBA. The relative quantum yield, radiative and non-radiative decay constants are calculated in alkanes and alcohols using the single point method. It is found that the quantum yield of the molecule varies from 16.81% to 50.79% with the change in solvent polarity, indicating the dependence of fluorescence on the solvent environment. Copyright © 2015 John Wiley & Sons, Ltd.

Triphenylamine substituted dipyrrinato metal complexes: Synthesis, optical and electrochemical studies

A series of triphenylamine substituted dipyrrinato metal complexes (1–8) have been synthesized. The mononuclear type complexes 1–6 have Ni(II), Co(II), Pd(II), In(III), and Zn(II) metal ions in the core. The binuclear type complexes 7 and 8 have Zn(II) metal ion in the center. All the compounds (1–8) were characterized by HRMS, NMR, IR, UV–vis absorption, cyclic voltammetry and fluorescence techniques. The presence of large electron rich triphenylamine moiety at dipyrrin ligands affected the spectral properties of complexes. Except Co(II) complex, other metal complexes exhibited blue shifted absorption maxima in UV–vis studies. The In(III) and Zn(II) metal complexes 4–6 showed red shifted emission maxima in fluorescence compared to their corresponding phenyl analogues. Complexes 3–8 exhibited good Stokes shifts in the range of 4600 to 7000cm−1 with reduced quantum yields. Singlet state lifetimes of complexes 3–8 were in the range of 2 to 4ns; also the decrease in radiative decay constants kr and the increase in non-radiative decay constants knr were in line with the quantum yield data. CV studies of complexes 1–8 showed anodic shifts in the oxidation potentials, suggesting that meso-triphenylamine group has affected the electronic properties of complexes by making them difficult to oxidize.

Estimation of Dipole Moments and Quantum Yield of 5-chloro-2-methoxyphenyl Boronic Acid in Different Solvents Environment.

The Photophysical properties like ground state and excited state dipole moments, change in the dipole moment and fluorescence quantum yield of a boronic acid derivative 5-chloro-2-methoxy phenyl boronic acid (5CMPBA) are characterized. The study is carried out in various solvents at room temperature using absorption and steady-state fluorescence technique. The emission wavelength of 5CMPBA is quite sensitive to the polarity of solvents. With the increase in solvent polarity red shift or bathochromic shift of about 9 nm has been observed. The excited state and ground state dipole moments are estimated using solvatochromic shift method and effect of solvents on spectral properties of the molecule are investigated using Kamlet-Taft multiple linear regression approach. The changes in dipole moment (Δμ) are calculated both from solvatochromic shift method and microscopic solvent polarity parameter (E T (N) ), and the values are compared. The ground state dipole moment is also evaluated using quantum chemical calculations. The bathochromic shift of the emission spectra and the increase in the excited state dipole moment indicates π → π* transitions as well as the possibility of intramolecular charge transfer (ICT) character in the emitting singlet state of 5CMPBA. The relative quantum yield (Φ), radiative and non-radiative decay constants are calculated using single point method. It is found that the quantum yield of the molecule varies from 11 to 64% with the change in the solvent polarity indicating the dependency of fluorescence nature on the solvent environment.

Shedding light on the photophysical properties of iridium(III) complexes with N-heterocyclic carbene ligands from a theoretical viewpoint.

The phosphorescent efficiencies of the Ir(III) carbene complexes 1-3 with wide-range color tuning were focused on in this work. A DFT/TDDFT (density functional theory/time-dependent density functional theory) investigation on the geometries in the ground and lowest triplet excited states, the frontier molecular orbitals, the absorption spectra, and d-orbital splittings of 1-3 were provided to get a better understanding of structure–property relationships. Importantly, to shed light on the difference in phosphorescent quantum yields for 1-3, radiative decay constants as well as zero-field-splitting parameters were calculated based on the estimation of spin–orbit coupling (SOC) matrix elements denoted as T(α1)|H(SOC)|S(n). The results show that, for any complex, the radiative decay rates in the three substates (namely, T(x), T(y), and T(z0) are not equal, and the largest radiative rates of 1-3 are all located in x substates with values of 1.0764 × 10(4), 0.8231 × 10(4), and 1.9596 × 10(4) s(–1), respectively. Moreover, for 3 with the highest quantum efficiency, we make efforts to modify it through varying substituents and substituent positions not only to achieve blue shift in the emission but also to obtain improved triplet energy.

A Luminescent Rhenium(I) Metallacycle with Oxygen and Nitrogen Donor Atoms: Photophysical and Theoretical Studies

AbstractA new N,O‐bischelator‐bridged dirhenium(I) metallacycle, [Re2(CO)6(μ‐η4‐mpdo)(μ‐4,4′‐tmdp)] (1, H2mpdo = 2‐methylpyrimidine‐4,6‐diol, 4,4′‐tmdp = 4,4′‐trimethylenedipyridine) has been self‐assembled in a one‐pot reaction in p‐xylene at 140 °C. Compound 1 displays unusual multiple phosphorescence emissions at room temperature; an emission quantum yield of 0.26 ± 0.03 and triexponential decay with an average radiative decay constant of 0.052 μs (λmax = 342 nm) were determined. These results as well as those of DFT/TDDFT calculations confirmed the nature of the emitting states.

Solvent effect on the relative quantum yield and fluorescence quenching of 2DAM

The relative quantum yield of diethyl 2-acetamido-2-((3-oxo-3H-benzo[f]chromen-1-yl)methyl)malonate [2DAM] is estimated using single point method with quinine sulfate as standard reference. The quantum yield varies between 0.1161 and 0.3181 depending on the nature of the solvent. The rates of radiative and non radiative decay constants are also calculated. The fluorescence quenching of [2DAM] by aniline is studied at room temperature, by steady state, in five different solvents namely acetonitrile (AN), 1,4 dioxane (DX), 1,2 dichloroethane (DCE), tetrahydrofuran (THF) and toluene (TOL), in order to explore various possible quenching mechanisms. The experimental results show a positive deviation in Stern Volmer plots for all solvents. Various parameters for the quenching process are determined by ground state complex, sphere of action static quenching model and finite sink approximation model. The magnitudes of these rate parameters indicate that positive deviation in the Stern Volmer (SV) plot is due to both static and dynamic processes. Further, finite sink approximation model is used to check whether these bimolecular reactions were diffusion limited or not. The values of distance parameter R′ and diffusion co efficient D are determined and then compared with the values of encounter distance R and diffusion coefficient D calculated using Stokes–Einstein equation.

Gray-Tone Lithography Implementation of Drexhage’s Method for Calibrating Radiative and Nonradiative Decay Constants of Fluorophores

We present a straightforward method to realize non-planar dielectric structures with a controlled height profile for use in calibration of fluorophores. Calibration of fluorescence quantum efficiency and intrinsic radiative and nonradiative decay rates of emitters is possible by using changes in the local density of optical states, provided one can control the emitter-surface distance with nanometer accuracy. We realize a method that is accurate yet fast to implement. We fabricate PMMA wedges (4 mm x 4 mm x 2 \mu m) by gray-tone UV-lithography of Shipley S1813G2. Its applicability as dielectric spacer is demonstrated in Drexhage experiments for three different emitters in the visible and near-infrared wavelength regime. The decay-rate dependence of the fluorescent state of emitters on the distance to a silver mirror is observed and compared to calculations based on the local density of states. Quantitative values for (non)radiative decay rates and quantum efficiencies are extracted. Furthermore, we discuss how Drexhage experiments can help to scrutinize the validity of effective material parameters of metamaterials in the near field regime.

POWER COUNTING OF VARIOUS DIRAC COVARIANTS IN HADRONIC BETHE–SALPETER WAVE FUNCTIONS FOR PSEUDOSCALAR MESON DECAYS

We have employed the framework of Bethe–Salpeter equation under covariant instantaneous ansatz to calculate leptonic decay constants of unequal mass pseudoscalar mesons like π±, K, D, DS and B, and radiative decay constants of neutral pseudoscalar mesons like π0 and ηc into two photons. In the Dirac structure of hadronic Bethe–Salpeter wave function, the covariants are incorporated from their complete set in accordance with a recently proposed power counting rule. The contribution of both leading order and next-to-leading order Dirac covariants to decay constants are studied. The results are found to improve and hence validating the power counting rule which provides a practical means of incorporating Dirac covariants in the Bethe–Salpeter wave function for a hadron.

Highly Luminescent Mixed-Metal Pt(II)/Ir(III) Complexes: Bis-Cyclometalation of 4,6-Diphenylpyrimidine As a Versatile Route to Rigid Multimetallic Assemblies

The proligand 4,6-di-(4-tert-butylphenyl)pyrimidine LH(2) can undergo cycloplatination with K(2)PtCl(4) at one of the two aryl rings to give, after treatment with sodium acetylacetonate, a mononuclear complex Pt(N^C-LH)(acac) (denoted Pt). If an excess of K(2)PtCl(4) is used, a dinuclear complex of the form [Pt(acac)](2){μ-(N^C-L-N^C)} (Pt(2)) is obtained instead, where the pyrimidine ring acts as a bridging unit. Alternatively, the mononuclear complex can undergo cyclometalation with a different metal ion. Thus, reaction of Pt with IrCl(3)·3H(2)O (2:1 ratio) leads, after treatment with sodium acetylacetonate, to an unprecedented mixed-metal complex of the form Ir{μ-(N^C-L-N^C)Pt(acac)}(2)(acac) (Pt(2)Ir). The mononuclear iridium complex Ir(N^C-LH)(2)(acac) (Ir) has also been prepared for comparison. The UV-visible absorption and photoluminesence properties of the four complexes and of the proligand have been investigated. The complexes are all highly luminescent, with quantum yields of around 0.5 in solution at room temperature. The introduction of the additional metal centers is found to lead to a substantial red-shift in absorption and emission, with λ(max) in the order Pt < Pt(2) < Ir < Pt(2)Ir. The trend is interpreted with the aid of electrochemical data and density functional theory calculations, which suggest that the red-shift is due primarily to a progressive stabilization of the lowest unoccupied molecular orbital (LUMO). The radiative decay constant is also increased. This versatile design strategy may offer a new approach for tuning and optimizing the luminescence properties of d-block metal complexes for contemporary applications.

Photophysical properties of an asymmetrical 2,5-diarylidene-cyclopentanone dye possessing electron donor and acceptor substituents

Spectroscopic and photophysical properties have been measured in a variety of solvents for (2E,5E)-2-(p-cyanobenzylidene)-5-(p-dimethylaminobenzylidene)-cyclopentanone ( I), an asymmetrically substituted 2,5-diarylidene-cyclopentanone known to have potential for applications utilizing two-photon absorption (TPA). Compound I was synthesized by a simple procedure utilizing DIMCARB catalyst. Absorption and fluorescence maxima have been correlated with the E T(30) solvent polarity scale. Theoretical TD-DFT calculations and Lippert–Mataga analysis demonstrate the internal charge transfer (ICT) nature of the S 0 → S 1 excitation. Radiative and nonradiative decay constants have been determined from fluorescence quantum yields and lifetimes. Substantial variation with solvent in the rate of nonradiative decay is interpreted in terms of a competition between internal conversion and intersystem crossing.

Ground- and Excited-State Dynamics of Aluminum and Gallium Corroles

The steady-state absorption and emission spectra and the temporal fluorescence decay profiles of two metallocorroles, Al(tpfc)(py)(n) and Ga(tpfc)(py)(n) (n = 1,2), have been measured in a noncoordinating solvent, benzene, in a coordinating solvent, pyridine, and in mixed benzene-pyridine solutions. The ground-state spectra reveal that an equilibrium between the pentacoordinate corrole (n = 1) and the hexacoordinate corrole (n = 2) is established in the mixed benzene-pyridine solutions. The ground-state equilibrium constants are 135 M(-1) and 1.0 M(-1) at 295 K for the Al and Ga species, respectively. The excited-state radiative and nonradiative decay constants of the pentacoordinate and the hexacoordinate species have been obtained from measurements of the fluorescence quantum yields and monoexponential fluorescence decay times in pure benzene and pure pyridine. Temporal fluorescence decays of the gallium system in a mixed benzene-pyridine solution are biexponential due to dissociation of the hexacoordinate species in the excited state leading to the establishment of a dissociation-association equilibrium. The rate constants for the pyridine association and dissociation processes for the gallium corrole in the excited state have been measured, k(a)* = 2.3 x 10(8) M(-1) s(-1) and k(d)* = 2.9 x 10(8) s(-1), respectively, leading to a value for the excited-state association equilibrium constant of K(a)* = 0.78 M(-1).

Photophysics and spectroscopy of the higher electronic states of zinc metalloporphyrins: a theoretical and experimental study

The photophysics of the S2 and S1 excited states of zinc porphyrin (ZnP) and five of its derivatives (ZnOEP, ZnTBP, ZnTPP, ZnTFPP, ZnTCl8PP) have been investigated by measuring their steady-state absorption and fluorescence spectra, quantum yields and excited state lifetimes at room temperature in several solvents. The radiative and radiationless decay constants of the fluorescent excited states accessible in the visible and near UV regions of the spectrum have been obtained. Despite the similarities in the Soret spectra of these compounds, their S2 excited state radiationless decay rates differ markedly. Although the S2-S1 electronic energies of a given zinc porphyrin vary linearly with the Lippert (refractive index) function of the solvent, the S2 radiationless decay rates of the set of compounds do not follow the energy gap law of radiationless transition theory. Calculations, using time-dependent density functional theory (TDDFT), of the energies and symmetries of the complete set of excited states accessible by 1- or 2-photon absorption in the near UV-visible have also been carried out. Substitution on the porphyrin macrocycle framework affects the ground state geometry and alters the electron density distributions, the orbital energies and the relative order of the excited electronic states accessible in the near UV-blue regions of the spectrum. The results are used to help interpret both the nature of the electronic transitions in the Soret region, and the relative magnitudes of the radiationless transition rates of the excited states involved.

Photophysics of pyrene-labelled compounds of biophysical interest

The effects of the chemical constitution and structure of the substituent on the excited state dynamics of several model fluorescent pyrene-labelled molecules of biophysical interest have been examined. Nine new 1-substituted pyrenyl compounds, Py-NH-CO-C2H5, Py-NH-CO-Leu-Boc, Py-CH2-NH-CO-C2H5, Py-CH2-NH-CO-Leu-Boc, Py-CO-NH-C3H7, Py-CO-NH-Leu-OMe, Py-CH2-CO-NH-C3H7, Py-CH2-CO-NH-Leu-OMe and Py-C3H6-CO-NH-Leu-OMe, have been synthesized and their electronic spectra, fluorescence quantum yields and excited state lifetimes measured. These data have been used to calculate the radiative, kr, and non-radiative decay constants of their S1 states and the values of these constants correlated with the structures of the tethers. Non-radiative S1 decay rates (mainly intersystem crossing to T1) vary in parallel with the radiative rates so that the excited state lifetimes and radiative rate constants change considerably with the structure of the substituent whereas the quantum yields of fluorescence do not. An excellent correlation between [epsilon]max of the S1-S0 transition and either kr or the excited state lifetime is observed as long as no additional intermolecular or intramolecular excited state decay process of significant rate competes with the 'normal' radiative and non-radiative (ISC) decay processes of the pyrenyl chromophore. This correlation may have predictive value. Rates of bimolecular quenching of the S1 states of these molecules by molecular oxygen have been measured. The quenching process is diffusion-controlled with a spin statistical factor of 1, indicating that the S1-T1 electronic energy spacings of all the derivatives exceed the O2(1Deltag-3Sigmag-) electronic excitation energy of ca. 1 eV.

Surface Eu3+ions are different than “bulk” Eu3+ions in crystalline doped LaF3nanoparticles

Distinct surface effects on the luminescence properties of highly crystalline LaF3:Eu nanoparticles have been demonstrated, by incorporating different ligands on their surface as well as forming core–shell nanoparticles (i.e. doped LaF3:Eu core with an undoped LaF3 shell). These studies have established that the surface Eu3+ ions have a less symmetric crystal field than the “bulk”, and that they are responsible for the ligand induced changes in the asymmetric ratio, which is determined from the relative intensity ratio of the 5D0 → 7F2 (612 and 618 nm) and 5D0 → 7F1 (591 nm) transitions. This surface effect can very effectively be reduced by forming core–shell particles. The multi-exponential luminescent decay curves observed for these nanoparticles have been fitted using a three-parameter model involving the radiative decay constant of the nine inner shells (kR), the radiative decay constant of the outermost shell (kR10), and a parameter C, which describes the quenching, and is a function of the particular luminescent lanthanide ion, the type and amount of quenchers on or near the surface. The model accounts well for the observed changes in the asymmetric ratio of luminescence of these nanoparticles brought about by the ligands used to stabilise them.

Dual probe solubilisation in two distinct regions of pure and mixed micelles: a pico-second time resolved fluorescence study

Photophysical parameters of a newly synthesised fluorescence probe, 2-[3-( N-methyl- N-phenyl amino)-2-propenylidene] indanone, have been studied in three different micellar media formed by the surfactants sodium dodecyl sulphate (SDS), cetyl trimethyl ammonium bromide (CTAB) and triton X-100 (TX100) and their binary mixtures (SDS–TX100, CTAB–TX100). Our study indicates that the probe molecules are distributed between the core and Stern/ palisade layer of the micelle. The extent of distribution depends on the nature of the micelle. The radiative decay constant ( k r) for the excited S 1 state has been found to be independent of the nature of micellar media. However, the non-radiative decay constant ( k nr) exhibits significant medium dependence in the order SDS > CTAB ≈ TX100. Non-linear variation of k nr with component surfactant composition in the mixed micelle has been observed and has been explained in terms of synergistic interaction between the surfactant molecules.

Polarium model: Reflection and transmission of coherent radiation

The coherent, radiative excitation modes of a resonant medium lend a resonant character to the amplitudes for reflection and transmission of light. For a plane wave incident upon a slab-shaped medium of two-level atoms, the reflection and transmission coefficients exhibit sharp peaks and dips as a function of the detuning of the wave from the central atomic resonance frequency. The locations and widths of these resonant features for a normally incident plane wave correspond to the imaginary and real parts, respectively, of the radiative decay constants of the coherent excitation modes of the medium discussed in the preceding paper [Phys. Rev. A 61, 063814 (2000)]. For the case of oblique incidence, generalizations of the modes discussed in the preceding paper are required. The richly detailed, resonant frequency dependence of the reflected and transmitted intensities is shown to depend significantly on the angle of incidence and the polarization of the incident light.