Synthesis and photophysical properties of polyamides containing in-chain porphyrin and [60]fullerene

Conjugated polyamides containing porphyrin and [60]fullerene (C₆₀) in the main chain were prepared by a direct polycondensation of the 3′H,3″H-dicyclopropa[1, 9:16, 17] [5, 6]fullerene-C₆₀-I _h -3′,3″-dicarboxylic acid and 5,15-bis(4-aminophenyl)-10,20-bis(3,5-dialkoxyphenyl)porphyrin in the presence of triphenyl phosphite and pyridine. Gel permeation chromatography (GPC) analysis of the polyamides showed the weight-average molecular weight was about 23,626–23,736, and the temperature at 5% weight loss determined by thermogravimetric analysis (TGA) was above 216 °C. The transmission electron microscopy (TEM) images displayed the regular one-dimensional linear arrays of the polyamides with lengths exceeded 200 nm. The photoinduced electron transfer from porphyrin to C₆₀ in the polyamides was observed in nanosecond laser-flash photolysis experiments at ambient temperature, which produced a charge-separated state (porphyrin radical cation–C₆₀ radical anion pair) with a lifetime as long as 40 μs. The calculated ratio of k _CS/k _CR was found to be 2.1 × 10⁴. They could have potential applications for photoelectronic devices, organic solar cells and so on.     

Time-resolved studies on the photoisomerization of a phenylene-silylene-vinylene type compound in its first singlet excited state

In femtosecond laser-flash photolysis experiments, the first singlet excited state of trans-ST, ((E,E)-{1,4-bis(2-dimethylphenylsilyl)ethenyl}benzene) showed a strong S1(π,π^?)-Sn absorption band at 540 nm in acetonitrile and at 550 nm in hexane. The lifetime of this state was determined to be 13.2±2.0 and 11.1±1.5 ps, respectively. Intersystem crossing was shown not to be a principal route for the deactivation of this S1 state of trans-ST. Evidence for this conclusion involved two complementary nanosecond laser-flash photolysis experiments. In one experiment involving direct excitation, no transient absorption spectrum was detected in the 350-650 nm spectral range. Yet, in the second experiment, on triplet sensitization, using xanthone, a transient absorption at 400 nm was tentatively assigned to the triplet state absorption of trans-ST. Photoisomerization was monitored in nanosecond time-resolved bleaching experiments. From these experiments the trans-cis photoisomerization quantum yield was determined to be 0.23 on direct trans-ST excitation. In a xanthone-sensitized stationary-state excitation experiment, the trans-cis isomerization quantum yield was determined to be 0.32. The main deactivation route of trans-ST in its S1 state is repopulation of the ground state directly through internal conversion or with the intermediacy of conformers with twisted geometry.     

Steady-state and time-resolved spectroscopic investigations on intramolecular electron transfer processes within a synthesized methoxynaphthalene dyad by using a nematic liquid crystal medium

UV-vis, steady state and time-resolved spectroscopic investigations were made on photoinduced charge separation and thermal charge recombination processes involved within a novel synthesized dyad, 1-(4-chloro-phenyl)-3-(4-methoxy-naphthalen-1-yl)-propenone (MNCA) where the donor 1-methoxynaphthalene (MNT) and the acceptor p-choloroacetophenone (PCA) moieties are connected by a short unsaturated olefinic bond. The measurements were made within the pseudo-ordered domain (just above nematic-isotropic (N-I) phase transition temperature, >308 K) of a nematic liquid crystal, 4-(n-pentyl)-4′-cyanobiphenyl (5CB). Results observed are compared with those obtained from the similar measurements in isotropic media. The charge separation and recombination rates remain more-or-less unchanged within the experimental error irrespective of the polarity of the environment, whether in pseudo-ordered domain (ε_S∼10.5) of a nematic liquid crystal 5CB or in highly polar isotropic medium ACN (ε_S∼37.5). The structural rigidity of the dyad MNCA having stable elongated form both in the ground as well as in the photoexcited states seems to be the reason for this unique behavior of solvent insensitivity. The theoretical predictions done by ab initio method density functional theory (DFT) with B3LYP/6-311 G (d, p) basis function correlate well with experimental observations of formations of only one stable elongated (E-type) conformer both in the ground and electronic excited state.     

Steady State Fluorescence Studies of Wild Type Recombinant Cinnamoyl CoA Reductase (Ll-CCRH1) and its Active Site Mutants

Fluorescence quenching and time resolved fluorescence studies of wild type recombinant cinnamoyl CoA reductase (Ll-CCRH1), a multitryptophan protein from Leucaena leucocephala and 10 different active site mutants were carried out to investigate tryptophan environment. The enzyme showed highest affinity for feruloyl CoA (K _a ?=?3.72?×?10⁵ M^?1) over other CoA esters and cinnamaldehydes, as determined by fluorescence spectroscopy. Quenching of the fluorescence by acrylamide for wild type and active site mutants was collisional with almost 100 % of the tryptophan fluorescence accessible under native condition and remained same after denaturation of protein with 6 M GdnHCl. In wild type Ll-CCRH1, the extent of quenching achieved with iodide (f _a?=?1.0) was significantly higher than cesium ions (f _a?=?0.33) suggesting more density of positive charge around surface of trp conformers under native conditions. Denaturation of wild type protein with 6 M GdnHCl led to significant increase in the quenching with cesium (f _a?=?0.54), whereas quenching with iodide ion was decreased (f _a?=?0.78), indicating reorientation of charge density around trp from positive to negative and heterogeneity in trp environment. The Stern-Volmer plots for wild type and mutants Ll-CCRH1 under native and denatured conditions, with cesium ion yielded biphasic quenching profiles. The extent of quenching for cesium and iodide ions under native and denatured conditions observed in active site mutants was significantly different from wild type Ll-CCRH1 under the same conditions. Thus, single substitution type mutations of active site residues showed heterogeneity in tryptophan microenvironment and differential degree of conformation of protein under native or denatured conditions.     

Temperature-dependent Time-resolved Fluorescence Study of Cinchonine Alkaloid Dication

Photo induced excited state dynamical processes of cinchonine alkaloid dication (C⁺⁺) have been studied over a wide range of temperature using steady state and nanosecond time-resolved fluorescence spectroscopic techniques. The temperature-dependent fluorescence studies of C⁺⁺ clearly indicate the existence of two distinct emitting species having their own characteristic decay rates. The shorter-lived species shows a usual temperature dependence with increasing non-radiative deactivation at higher temperatures, while the longer-lived species show features resembling to the excited state solvent relaxation process with a large solvent relaxation time (τ _r ∼ 6 ns). The species emitting in the lower energy side, having longer decay time is found to be more sensitive towards chloride ion quenching and has a charge transfer character. Further, concentration quenching with decrease in τ _r of long lived species shows the possibility of energy migration along with solvent relaxation in C⁺⁺.     

Improved luminescent properties of red-emitting Ca0.54Sr0.16Eu0.08Gd0.12(MoO4)0.2(WO4)0.8 phosphor for LED application by charge compensation

The red-emitting Ca_0.54Sr_0.16Eu_0.08Gd_0.12(MoO₄)_0.2(WO₄)_0.8 phosphor is improved in the emission charateristics by charge compensation, of which chromaticity coordinates (CIE) are x=0.66 and y=0.33. Three approaches to charge compensation are investigated, namely (a) 3Ca²⁺/Sr²⁺→2Eu³⁺/Gd³⁺+vacancy, (b) 2Ca²⁺/Sr²⁺→Eu³⁺/Gd³⁺+M⁺(M⁺ is a monovalent cation like Li⁺, Na⁺ and K⁺ employed as a charge compensator) and (c) Ca²⁺/Sr²⁺→Eu³⁺/Gd³⁺+N⁻ (N⁻ is a monovalent anion like F⁻, Cl⁻, Br⁻ and I⁻ employed as charge compensation ions). Through photoluminescent spectra analyzing the radiative and non-radiative relaxation mechanisms of luminescent system are obtained. Under 20 mA forward-bias current, one red-emitting LED is made by combining 390-405 nm-emitting LED chip and the phosphor. The red-emitting phosphor has broad prospects in LED application field.     

Photoinduced electron transfer within a novel synthesized short-chain dyad

The investigations were made by using electrochemical, steady state and time resolved spectroscopic (time correlated single photon counting and laser flash photolysis) techniques on a novel synthesized dyad, 1-(4-chloro-phenyl)-3-(4-methoxy-naphthalen-1-yl)-propenone (MNCA) where the donor 1-methoxy-naphthalene (MNT) is connected with the acceptor p-chloroacetophenone (PCA) by an unsaturated olefinic bond. This dyad possesses mainly extended (E-type) conformation both in the ground and excited state. The unchanged conformational geometry of this dyad even after photoexcitation makes it different from the previously studied benzothiophene-p-chloroacetophenone dyads, though both the donors present are isosteric molecules. In the cases of the latter dyads though E-type isomeric structure dominates in the ground state, considerable amount of Z-type (folded) species are produced in the excited state. It is hinted that the proximity effect of methoxy functionality in donor moiety, may be the reason for the formation of mostly E-isomeric species in the case of the present dyad MNCA system. The observed unchanged values of charge recombination and ion-pair lifetime, estimated from the analysis of transient absorption spectra of the dyad in presence of βCD and without it, confirm the proposition about the maintenance of the extended conformation even upon photoexcitation. From the transient absorption measurements it appears that due to increase of delay times between the exciting and probe pulses, higher triplet T_n of the donor being generated gets involved in PET reactions with the surrounding medium ACN. From the present findings, MNCA in aqueous medium seems to be better candidate to build light energy conversion devices than the previously studied benzothiophene dyads where artificial devices like βCD were used to make elongated geometry. This geometry would help to prevent charge recombination processes within the redox components due to minimal overlapping between them.     

Interplay between charge and antiferromagnetic ordering in Bi0.6−xPrxCa0.4MnO3 (0≤x≤0.6) perovskite manganite

Structure, magnetic and transport properties of polycrystalline Bi_0.6−xPr_xCa_0.4MnO₃ (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) have been studied. Systematic substitution of Pr at Bi site induces an interesting interplay between the charge ordering and antiferromagnetism. The charge ordering temperature (T_CO) decreases with increasing x. The antiferromagnetic (AFM) ordering temperature (T_N) increases sharply at both the extremes but remains nearly constant from x=0.2 to 0.4. At temperatures lower than T_N a transition to the glassy state is observed. The nature of this glass like state appears to be controlled by the Pr content, and at lower values of x this is akin to a spin glass, while at higher x it has a characteristic of cluster glass. The Pr doping also leads to enhancement in the magnetic moment. In the present work it has been proposed that the local lattice distortion induced due to size mismatch between the A-site cations and 6s² character of Bi³⁺ lone pair electron is responsible for the observed magnetic and electrical properties.     

Influence of structural dynamics on charge recombination rates in photogenerated radical ion pairs: Evidence from EPR spectroscopy and computation

In order to better understand the dependence of charge recombination rate vs. temperaturek _CR(T) within a linear donor-chromophore-acceptor (D-C-A) molecular triad, the structural dynamics of the cation radical D⁺-C is studied individually using variable-temperature electron paramagnetic resonance (EPR) spectroscopy and electronic structure calculations. Here, the donor D isp-methoxyaniline, the chromophore C is 4-(N-piperidinyl)-naphthalene-1,8-dicarboximide, and the acceptor A is naphthalene-1,8∶4,5-bis(dicarboximide). The EPR spectra of D⁺-C exhibit marked changes in their overall shape throughout the 190–295 K temperature range. These spectra have hyperfine splittings that are strikingly well simulated with a model that includes methoxy group rotation, which occurs at a rate of 2.6 · 10⁴ s^?1 at 210 K and speeds up to 1.25 · 10⁷ s^?1 at 295 K, corresponding to an energy barrier of 38 kJ/mol. This considerable barrier reflects the partial conjugation between MeO and the aromatic ring and is confirmed by the calculated energy of a series of D^{+ ·}-C rotamers. The simulations also reveal that inversion of the anilino N center emerges atT > 250 K and can be represented by a planar and a pyramidal conformation with the equilibrium constantK = [pyramidal]/[planar] increasing from 0.029 at 250 K to 0.56 at 295 K. In the same temperature range, the charge recombination rate of D^{+ ·}-C-A^{? ·} accelerates abruptly and can be separated into two components, according to the above planar/pyramidal equilibrium. Thek _CR (T) of the pyramidal conformation has an activation energy of 41 kJ/mol, virtually the same as the barrier of MeO rotation. These results show that the intramolecular structural dynamics of the radical cation within D^+·-C-A^{? ·} control the overall charge recombination reaction with this radical ion pair.     

Photoinduced interaction between MPA capped CdTe QDs and certain anthraquinone dyes

Photoinduced interaction of mercapto propionic acid (MPA) capped CdTe quantum dots (QDs) with certain anthraquinone dyes namely alizarin, alizarin red S, acid blue 129 and uniblue has been studied by steady state and time resolved fluorescence measurements. Addition of anthraquinone dyes to CdTe QDs results in the reduction of electron hole recombination has been observed (i.e., fluorescence quenching). The Stern-Volmer constant (K_SV), quenching rate constant (k_q) and association constants (K) were obtained from fluorescence quenching data. The interaction of anthraquinone dyes with QDs occurs through static quenching was confirmed by unaltered fluorescence lifetime. The occurrence of electron transfer quenching mechanism has been proved by the negative free energy change (ΔG_et) obtained as per the Rehm-Weller equation.     

Channeling radiation and coherent bremsstrahlung

We show that calculations based on the use of Bloch functions in at least two-dimensions predict both coherent brems-strahlung peaks (k_CB) and channeling radiation peaks (k_CR) in the spectrum of photons emitted by electrons traversing a crystal when moving nearly parallel to a crystal plane; and that furthermore, “sidebands” (e.g., k_CB ± k_CR) are predicted when the electrons move nearly parallel to two intersecting crystal planes.     

Photoinduced electron transfer in solutions of A Pd-porphyrin-quinone complex

Photoinduced electron transfer (PET) in a Pd-porphyrin-quinone complex (Pd-P-Q) was investigated using the flash photolysis method in microsecond and picosecond range and by luminescence. The investigations were performed for toluene solutions. Intramolecular PETs in the lower excited singlet state (k_CT=1.7·10¹⁰ sec⁻¹) and in the triplet state (k_CT=2.5·10⁸sec⁻¹) were observed. For a quantum yield of the triplet state that is close to 1 in Pd-porphyrin (Pd-P) this yield is equal to 0.4 in Pd-P-Q. This decrease is the result of PET in the excited state and, possibly, in an unrelaxed triplet state. Triplet Pd-P-Q molecules were efficiently quenched by unexcited molecules, and this process is related to intermolecular charge transfer. This electron transfer is likely to occur from the Pd-P protion of an excited molecule to the quinone protion of an unexcited molecule in the collisional complex (Pd-P-Q)₂. Charged radicals, formed once the collisional complex is transferred and separated, have a wide absorption band with a maximum of about 960 nm. The average lifetime of the radicals was about 2 msec. Institute of Molecular and Atomic Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No 1 pp. 61–70, January–February, 1997.     

Photophysical parameters and fluorescence quenching of 7-diethylaminocoumarin (DEAC) laser dye

The optical properties including electronic absorption spectrum, emission spectrum, fluorescence quantum yield, and dipole moment of electronic transition of 7-diethylaminocoumarin (DEAC) laser dye have been measured in different solvents. Both electronic absorption and fluorescence spectra are red shifted as the polarity of the medium increases, indicating that the dipole moment of molecule increases on excitation. The fluorescence quantum yield of DEAC decreases as the polarity of solvent increases, a result of the role of solvent polarity in stabilization of the twisting of the intramolecular charge transfer (TICT) in excited state, which is a non-emissive state, as well as hydrogen bonding with the hetero-atom of dye. The emission spectrum of DEAC has also been measured in cationic (CTAC) and anionic (SDS) micelles, the intensity increases as the concentration of surfactant increases, and an abrupt change in emission intensity is observed at critical micelle concentration (CMC) of surfactant. 2×10⁻³ mol dm⁻³ of DEAC gives laser emission in the blue region on pumping with nitrogen laser (λ_ex=337.1 nm). The laser parameters such as tuning range, gain coefficient (α), emission cross section (σ_e), and half-life energy have been calculated in different solvents, namely acetone, dioxane , ethanol, and dimethyforamide (DMF). The photoreactivity of DEAC has been studied in CCl₄ at a wavelength of 366 nm. The values of photochemical yield (?_c) and rate constant (k) are determined. The interaction of organic acceptors such as picric acid (PA), tetracyanoethylene (TCNE), and 7,7,8,8-tetracynoquinonedimethane (TCNQ) with DEAC is also studied using fluorescence measurements in acetonitrile (CH₃CN); from fluorescence quenching study we assume the possible electron transfer from excited donor DEAC to organic acceptor forming non-emissive exciplex.     

Short-living absorption and emission of CsI(Na)

This paper investigates the short-living absorption and the emission of CsI(Na) under a pulsed electron beam (Е_e=0.25 MeV, t_1/2=15 ns and W=0.003…0.16 J/cm²). The bands of singlet self-trapped excitons, as well as Na⁰ and V_k color centers have been detected in the transient absorption spectrum of CsI(Na). It has been found that the activator luminescence spectrum, peaking at 3.0 eV, fits a Gaussian (E_m=3.0 eV and FWHM=0.44±0.02 eV at 80 K) and remains the same at different time delays within 10⁻⁸-10⁻³ s. The decay kinetics of the 3.0 eV emission has one nanosecond exponential component and two microsecond ones with time constants 1.0 and 3.0 μs, which remain unchanged within 78-150 K. It is concluded that the activator emission is due to the radiative annihilation of sodium-perturbed two halide excitons from the non-relaxed singlet state. The pathways of such excitons creation are discussed.     

Kinetics and thermochemistry of the addition of atomic chlorine to acetylene

Atomic Cl was generated by pulsed laser photolysis at 193 nm of CCl₄, and was monitored by time resolved resonance fluorescence in the course of reaction with excess C₂H₂, diluted in Ar bath gas at pressures from 13 to 800 mbar. At 288 K simple pseudo first order kinetics were observed. Over 365-430 K bi-exponential decays were obtained, because of equilibration between the β-chlorovinyl adduct and the reactants. The ratios of forward and reverse rate constants yield Δ_fH₂₉₈(CHCHCl) = 274.0 ± 1.0 kJ mol⁻¹ via a Third-Law analysis of the carbon-chlorine bond strength. The thermochemistry compares well with that predicted by an initio theory. The effective second-order rate constant was pressure dependent and was analyzed using Troe’s unimolecular formalism. Over the whole temperature range the low-pressure limiting value for addition, with Ar bath gas, is given by k₀ = 4.1 × 10⁻³⁰ (T/300 K)^−2.47 cm⁶ molecule⁻² s⁻¹.     

Photophysical properties of substituted intramolecularly hydrogen bonded compounds: A combined experimental and theoretical study

Photophysical properties of prototype excited state intramolecular proton transfer (ESIPT) system 4-methyl-2,6-diformyl phenol (MFOH) and its derivatives were studied by steady state and time-resolved fluorescence spectroscopy as well as by ab-initio quantum chemical calculation. It has been found that nonradiative decay process is the most important deactivation channel in all the cases and the hydrogen bonded enol conformer is stable in the ground state whereas, the proton transferred keto form is energetically favoured in the S₁(ππ^*) state. However, the net gain in stabilization in the process of ESIPT is almost unaffected by the substitution. The reversal of stability in the excited state was explained on the basis of the nature of frontier molecular orbital in all the cases. Intrinsic reaction coordinate analysis showed that drastic change in nonbonded interoxygen distance R(O-O) in the proton transfer pathway causes the switch over from the enol to keto configuration. A close comparison of several properties like molecular geometry, hydrogen bond strength and atomic charge in different derivatives of MFOH were found to be consistent and in good agreement with the experimental results obtained from time-resolved fluorescence experiments.     

Influence of Mg on luminescence efficiency and charge compensating mechanism in phosphor CaAl12O19:Mn

Synthesized by a modified solid state method in air and mixed with MgO, the red phosphor of CaAl₁₂O₁₉:yMn⁴⁺ (y=0.001-1.5%) enhanced its photoluminescence efficiency by 3.5 times. The influence of MgO on crystal phases, luminescence intensity and spectral characteristics of the composition modified phosphor has been investigated by spectroscopic experiments and luminescence decay measurements. It is observed that the decay time of Mn⁴⁺ luminescence prolongs linearly with increase of MgO in the composition, indicating that the excitation energy transfer and non-radiative relaxation between Mn⁴⁺-Mn⁴⁺ pairs decrease. The presence of Mg²⁺ leads to a transformation of Mn⁴⁺-Mn⁴⁺ pairs connected with interstitial O²⁻ to isolated Mn⁴⁺ ions and therefore eliminates energy transfer and provides charge compensation as well.     

Enhanced performance of bi-layer Nb2O5 coated TiO2 nanoparticles/nanowires composite photoanode in dye-sensitized solar cells

Dye sensitized solar cells (DSSCs) were fabricated based on coumarin NKX-2700 dye sensitized bi-layer photoanode and quasi-solid state electrolyte sandwiched together with cobalt sulfide coated counter electrode. A novel bi-layer photoanode has been prepared using composite mixtures of 90 wt.% TiO₂ nanoparticles + 10 wt.% TiO₂ nanowires (TNPWs) as active layer and Nb₂O₅ is coated on the active layer, which acts as scattering layer. Hafnium oxide (HfO₂) was applied over the TNPWs/Nb₂O₅ photoanode film, as a blocking layer. TiO₂ nanoparticles (TNPs), TiO₂ nanowires (TNWs) and TNPWs/Nb₂O₅ were characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The sensitizing organic dye coumarin NKX-2700 displayed maximum absorption wavelength (λ_max) at 525 nm, which could be observed from the UV–vis spectrum. DSSC-1 fabricated with composite bi-layer photoanode revealed enhanced photo-current efficiency (PCE) as compared to other DSSCs and illustrated photovoltaic parameters; short-circuit current J_SC = 18 mA/cm², open circuit voltage (V_OC) = 700 mV, fill factor (FF) = 64% and PCE (η) = 8.06%. The electron transport and charge recombination behaviors of DSSCs were investigated by electrochemical impedance spectra (EIS) and the results illustrated that the DSSC-1 showed the lowest charge transport resistance (R_tr) and the longest electron lifetime (τ_eff). Therefore, in the present investigation, it could be concluded that the novel bi-layer photoanode with blocking layer increased the short circuit current, electron transport and suppressed the recombination of charge carriers at the photoanode/dye/electrolyte interface in DSSC-1.     

Spectral characteristics of 2-(4′-amino-3-pyridyl)benzimidazole: Effects of solvent and acid or base concentration

Spectral characteristics of 2-(4′-amino-3-pyridyl)benzimidazole (4-A3PyBI) have been studied in different solvents, as well as at different acid or base concentrations using absorption, fluorescence excitation and fluorescence spectroscopy. Excited singlet state (S₁) lifetimes for each species were measured using nanosecond time-dependent spectrofluorimeter. AM1 semi-empirical and density functional theoretical (DFT) calculations were performed on each species for the spectral assignment. From the above results it is concluded that 4-A3PyBI exists only in the amine form. First protonation occurs at pyridineN- atom and second protonation at the benzimidazole (BI)N- atom. When dication (DC) species is excited, two emission bands are observed, having the same fluorescence excitation spectra, suggesting the same ground state (S₀) precursor. Short wavelength (SW) emission band is assigned to the π-π* transition and long wavelength (LW) emission to the charge transfer transition. First deprotonation in S₀ state occurs from >N-H moiety, whereas in S₁ state it is from -NH₂ group. Monoanion (MA) so formed in S₁ state is non-fluorescent. Dianion (DA) is formed by further deprotonating >N-H moiety in S₁ state and it is fluorescent. pK_a values were determined and discussed.