Spectroscopic investigations of a novel push-pull azo compound embedded in rigid polymer

A new heterocyclic push-pull azo compound-in-poly(methymethacrylate) (PMMA) film has been made by means of the spin-coating method. The spectroscopic properties of the films have been investigated with the steady-state absorption spectra, and steady-state fluorescence and femtosecond time-resolved fluorescence spectra in the first time, which is an important characteristic for the application of the film. The excited singlet (S₁) state lifetimes for trans and cis isomers of the film at room temperature have been measured. The excited triplet (T₁) state lifetime of cis isomer of the film has been obtained. The electronic structure of the film has been explained. The results show that the aggregate state of the azo molecules greatly influences its absorption spectra.     

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.     

Electron-phonon and vibronic couplings in the FMO bacteriochlorophyll a antenna complex studied by difference fluorescence line narrowing

The electron-phonon and vibronic couplings governing the spectral properties have been studied in the Fenna-Matthews-Olson (FMO) bacteriochlorophyll a (BChl a)-protein complex at 4.5 K using a spectrally selective difference fluorescence line-narrowing technique. The complex is a part of the light-harvesting system of green photosynthetic bacteria. Its lowest-energy absorption band, peaking at 826 nm and responsible for the fluorescence, is believed to be due to Q_y transitions of largely isolated molecules. One of the main merits of the used method compared with the more common fluorescence line narrowing is that the zero-phonon lines (ZPL) resonant with the excitation laser can be accurately measured, allowing precise determination of the Huang-Rhys (HR) factors, the main characteristics of the linear electron-phonon and vibronic coupling strengths. Over 60 individual vibrational modes of intra- and intermolecular origin have been resolved in the energy range of 45-1600 cm⁻¹. The small HR factors for these modes, ranging between 0.001 and 0.018, add up to a value of S_vib=0.38±0.07. The effective HR factor for the phonons, S_ph, was found clearly wavelength-dependent, varying from ∼0.7 at short wavelengths to ∼0.3 at the long-wavelength tail of the absorption spectrum. Coupling between the BChl a molecules is likely responsible for this wavelength dependence.     

Role of various LB parameters on the optical characteristics of mixed Langmuir-Blodgett films

This communication reports the spectroscopic characterizations of mixed Langmuir-Blodgett (LB) films of non-amphiphilic N,N-bis (2,5-di-tert-butylphenyl)- 3,4,9-perylenedicarboximide (DBPI) molecules, mixed with polymethyl methacrylate (PMMA) and stearic acid (SA). J- aggregates of DBPI molecules in the mixed LB films have been confirmed by UV-Vis absorption spectroscopic study. Formation of organized structure of molecular stacking in the mixed LB films gives rise to the strong excimeric emission, which is manifested by a broad structureless band in the longer wavelength region of the fluorescence spectra and is confirmed by excitation spectroscopic study. A weak hump at around 576 nm due to monomeric emission is observed in the fluorescence spectra of 0.1 M of DBPI-PMMA mixed LB films of lower number of layers. The intensity of the 0-0 band at 530 nm in the fluorescence spectra is observed to be a function of the molefraction, number of layers, surface pressure of lifting and the matrix materials.     

Radiative lifetime measurements in neutral zirconium using time-resolved laser induced fluorescence in supersonic free-jet

Radiative lifetimes of 20 odd-parity excited energy levels in Zr I, out of which 10 for the first time, have been investigated in the energy range 17 400-29 300 cm⁻¹. The levels belong to the 4d²5s5p and 4d³5p electronic configurations. Time-resolved laser-induced fluorescence spectroscopy technique in supersonic free-jet has been employed in the present work to circumvent the effects of collision in lifetime measurements.     

Growth and optical property of ZnWO4: Er crystal

Good quality crystals ZnWO₄ activated with Er³⁺ have been grown by means of Czochralski method and characterized using optical spectroscopy techniques. XRD, absorption spectra, fluorescence spectrum are presented and the Judd-Ofelt intensity parameters Ω₂, Ω₄, and Ω₆ are obtained to be 6.76×10^-20, 0.37×10^-20, and 0.50×10^-20 cm², respectively. Along crystallographic axes, refractive indices are presented. The fluorescence decay time of the ⁴I_13/2 level has also been investigated and shows an exponential behavior with a lifetime value of 5.52 ms. The crystal is potentially used for green and infrared eye-safe lasers.     

Analysis of the low temperature fluorescence and phosphorescence spectra of p-difluorobenzene

The low temperature fluorescence spectra (4.2 K) of solid solutions of p-difluorobenzene-h₄ (pDFB-h₄) and -d₄ have been recorded and analyzed. The absence of fluorescence from vibrationally excited states at 4.2 K eliminates the sequence and “hot band” structure which complicate the analysis of the vapor fluorescence spectrum. On the basis of a comparison of our vibrational analysis with those published for the vapor phase fluorescence, several incorrect assignments in the latter have been identified. The high resolution and lack of spectral congestion obtained in the low temperature matrix isolated fluorescence spectra also allowed additional vibrational assignments to be made. These results are of added importance because of the extensive use which has been made of pDFB for radiationless relaxation studies in the vapor phase. The phosphorescence spectrum of crystalline pDFB-h₄ was observed with sufficient intensity to locate the electronic origin to make several vibrational assignments. Additionally, the zero-field splitting of the first triplet excited state was measured by optically detected magnetic resonance techniques.     

Photophysical characterizations of 2-(4-biphenylyl)-5 phenyl-1,3,4-oxadiazole in restricted geometry

Langmuir and Langmuir-Blodgett (LB) films of non-amphiphilic 2-(4-Biphenylyl)-5-phenyl-1,3,4-oxadiazole (abbreviated as PBD) mixed with stearic acid (SA) as well as with the inert polymer matrix poly(methyl methacrylate) (PMMA) have been studied. Surface pressure versus area per molecule (π-A) isotherm studies suggest that PBD molecules very likely stand vertically on the air-water interface and this arrangement allows the PBD molecules to form stacks and remain sandwiched between SA/PMMA molecules. At lower surface pressure, phase separation between PBD and matrix molecules occurs due to repulsive interaction. However, at higher surface pressure, PBD molecules form aggregates. The UV-vis absorption and steady-state fluorescence spectroscopic studies of the mixed LB films of PBD reveal the nature of the aggregates. H-type aggregate predominates in the mixed LB films, whereas I-type aggregate predominates in the PBD-PMMA spin-coated films. The degree of deformation produced in the electronic levels are largely affected by the film thickness and the surface pressure of lifting.     

Local variation of the dielectric properties of poly(vinylidene fluoride) during the α- to β-phase transformation

The phase transformation from the non-polar α-phase to the polar electroactive β-phase of polyvinylidene fluoride (PVDF) has been investigated using the fluorescence from Nile red. Films of α-PVDF doped with Nile red were stretched at controlled rates at a temperature of 80 °C to produce the α- to β-phase transition. The thermo/mechanical dependent changes in the crystalline structure are related to the physical rotation of the polar (CH₂-CF₂) group, which can be monitored by steady state fluorescence techniques. The degree of phase transformation is related to variation in the fluorescence, which in turn is linked to local dielectric constant of the polymer. The variation of the refractive index is more associated to the alignment of the polymeric chains than to the phase transformation. Thus, fluorescence is a suitable technique to monitor phase transitions coupled to a variation in the polarity of the dielectric medium.     

Energy-transferred photoluminescence from thiophene/phenylene oligomer thin films

Photoluminescence (PL) based on Förster energy transfer between p-sexiphenyl (p-6P) and 5,5′-bis(4-phenylyl)-2,2′-bithiophene (BP2T) was investigated for their coevaporated and laminated thin films. In the former films, fluorescence quenching of the p-6P was accompanied by appearance of BP2T fluorescence, which indicated existence of the energy transfer between the donors and the acceptors. The latter films were fabricated by successive depositions of p-6P, MgF₂ and BP2T in which the thickness of the MgF₂ spacer was varied. The energy-transferred acceptor fluorescence was suppressed by the spacer thicker than the Förster distance (∼10 nm).     

7-Hydroxy-4-methyl-8-(4′-methylpiperazine-1′-yl)methyl coumarin: An efficient probe for fluorescence resonance energy transfer to a bioactive indoloquinolizine system

Solvatochromic effects on the fluorescence behavior of 7-hydroxy-4-methyl-8-(4′-methyl-piperazine-1′ yl)methylcoumarin (HMMC) was studied in different solvents. The fluorescence of HMMC was found to be highly sensitive to both the polarity and the protic character of the solvent. Exploiting the polarity-sensitive fluorescence property of HMMC, its excited-state dipole moment has been determined. Fluorescence (Förster) resonance energy transfer (FRET) process from HMMC to a potent bioactive molecule 3-acetyl-4-oxo-6,7-dihydro-12 H indolo-[2,3-a] quinolizine (AODIQ) was studied. From the determined K_SV and R₀ values, it is argued that a long-range dipole-dipole interaction is operating for the energy transfer mechanism. The energy transfer efficiency (E) and the distance between the acceptor and the donor (r₀) have been determined.     

Synthesis and two-photon optical characterization of D-π-D type Schiff bases

The title compounds, several Schiff bases with D-π-D type have been successfully synthesized from reaction of 4-(N-dialkylamino) benzaldehyde and hydrazine or p-phenylenediamine in ethanol. These compounds were characterized by IR, ¹H NMR, elemental analysis and electronspray mass spectrometry. One-photon fluorescence and two-photon fluorescence properties have been investigated. The two-photon absorption (TPA) cross section (σ) of these compounds was obtained by using the open-aperture Z-scan technique. Based on theoretical calculations and factual test, the influence that a series of substituted groups and the conjugated frame imposed on TPA cross section were systematically discussed.     

Flourimetric and prototropic studies on the inclusion complexation of 2-amino and 4-aminodiphenyl ethers with β-cyclodextrin: Unusual behavior of 4-aminodiphenyl ether

The fluorescence characteristics of diphenyl ether (DPE), 2-aminodiphenyl ether (2ADPE) and 4-aminodiphenyl ether (4ADPE) and prototropic behavior of 2ADPE and 4ADPE on inclusion complexation with β-cyclodextrin have been investigated. DPE forms 1:1 complex whereas 2ADPE and 4ADPE form 1:2 complex with β-CDx. The fluorimetric and prototropic behaviors of 4ADPE in β-CDx are different from those in aqueous solution. The dual fluorescence of 4ADPE in β-CDx is found to be due to twisted intramolecular charge transfer (TICT) character induced by inclusion complexation. The two equilibria viz. monocation↔monocation solvent exciplex↔neutral reported for 4ADPE in aqueous solution are not observed in presence of β-CDx. The ground and excited state pK_a values for monocation-neutral equilibrium of 2ADPE and 4ADPE have been reported.     

Photon distribution function in blinking fluorescence of individual molecules

A photon distribution function w_N(T) for blinking fluorescence with bright on- and dark off-intervals is derived. The function w_N(T) is expressed via few Poissonian functions each of which relates to corresponding exponential process in quantum dynamics of a given individual molecule. The distribution of photons is calculated for short, middle and long time intervals as compared to off-intervals. The distributions are much broader than Poissonian distribution and have rather complicated shape. If time resolution of an experiment does not permit us to see off-interval and, therefore, fluorescence looks like CW emission, the distribution of photons gives a signal about existence of hidden off- intervals in such CW fluorescence.     

Durees de vie des niveaux 1D2 ET 1S0 du magnesium

Radiative lifetimes of the excited n¹S₀ (n = 4, 5) and n¹D₂ (n = 3, 4) states in magnesium have been measured by time resolved fluorescence technique using two photon excitation.     

Effect of structure and concentration of polymer, metal ion and pH of the medium on the fluorescence characteristics of hyperbranched polyamines

A series of hyperbranched polyamines have been prepared by the reported method. All these polyamines exhibit fluorescence at about 350-650 nm with maximum intensity at about 450 nm. The study shows that the fluorescence intensity and the range of wavelength of fluorescence are strongly influenced by the structure of the hyperbranched polyamines. The effect of concentration of the polymer, pH of the medium and metal ion has also been investigated on the fluorescence characteristics of sulfone containing hyperbranched polyamine (Ps), as it shows the best result among the studied polymers. The results show that the intensity of fluorescence increases with the decrease of concentration (5-0.1 g/L in N,N′-dimethyl sulfoxide) of polymer and with the increase of pH (3.1-11.6) of the medium. The quenching of fluorescence increases with the increase of concentration of Cu²⁺ ions (0.01-0.04%). The hyperbranched polyamine (Ps) has also been end capped with benzoyl chloride and 4-hydroxybenzaldehyde to study the influence of end groups. The results showed that the structure of end capped compounds has prominent role to influence fluorescence characteristics of this hyperbranched polyamine.     

Structure and magnetic properties of Fe4/Cun (n=2,4) superlattices from first-principles study

The structures and magnetic properties of Fe4/Cun (n=2, 4) superlattices have been investigated by the first-principles pseudopotential plane-wave method based on spin density approximation. Compared with the ideal fcc-Cu bulk structure, for the optimized Fe4/Cu2 model, obvious contraction of interlayer distances occurs on the interior Fe layers, whereas the interlayer distances of Fe layers in Fe4/Cu4 are expanded. The anti-parallel alignment magnetic moment and negative polarization of the interior Fe layer have been found in the Fe4/Cu2 model. This can be explained in terms of the magnetic-volume effect, and the moment of anti-parallel alignment attributes to the contracted interlayer distances between the interior Fe layers. The MR ratio has also been evaluated by means of the two-current model. The MR ratio of the Fe4/Cu2 model (4.89%) is much small than that of the Fe4/Cu4 one (23.65%).     

The emission and absorption spectra of chromyl chloride in an argon matrix at 4 K

The emission and absorption spectra of chromyl chloride (and the pure ³⁵Cl isotopic species) have been obtained in an argon matrix at 4 K. In emission, two systems are obtained and these correlate with the two systems previously found in the absorption spectrum of the pure crystal and which were analyzed as the lowest singlet and triplet systems. The emission systems have, therefore, been assigned as phosphorescence and fluorescence.     

Theoretical study on liquid crystal cyanobiphenyls: Phase stability and phase behavior

This article presents a theoretical study on liquid crystalline materials in homologous series of 4'-n-alkyl-4-cyanobiphenyl (nCB) with propyl (3CB), pentyl (5CB), and heptyl (7CB) groups. The atomic net charge and dipole moment components at each atomic center have been evaluated using the complete neglect differential overlap (CNDO/2) method. The modified Rayleigh–Schrodinger perturbation theory along with the multicentered-multipole expansion method has been employed to evaluate the long-range intermolecular interactions, while a ‘6-exp’ potential function has been assumed for short-range interactions. Further, these interaction energy values have been used as input to calculate the translational entropy, and free energy of nCB (n=3, 5, and 7) molecules during the stacking, and in-plane interactions. The observed results have been correlated with the mesogenic behavior and phase stability based on the thermodynamic parameters introduced in this article. Further, an attempt has been made to elucidate the flexibility of a configuration at a particular temperature, which has a direct relation with phase transition property of the molecules.     

New measurements of the emission spectra of Sm2+ in KMgF3 and NaMgF3 crystals

The fluorescence of divalent samarium in KMgF₃ and NaMgF₃ crystals is investigated. The emission is observed to originate from transitions between the ⁵D_J, and ⁷F_J multiplets of the 4?⁶ configuration. More precisely, the lowest ⁵D_J level, ⁵D₀, appears to be the most efficient emitting level in the temperature range 4–300K. Contrary to what has been reported elsewhere, the Sm²⁺ fluorescence in both crystals does not exhibit any broad band emission even at room temperature. The great number of lines in the ⁵D₀→⁷F_J patterns gives evidence of the multiple-center origin of the fluorescence.