Steady-state Fluorescence Spectra Research Articles

Energy Transfer from CdSe Quantum Dots to Porphyrin via Two-Photon Excitation

CdSe quantum dots have been used as energy donors to activate meso-tetra (4-sulfonatophenyl) porphine dihydrochloride. Pulses of 130 fs duration at a wavelength of 800 nm are used as the two-photon excitation light source. After excitation, steady-state and time-resolved fluorescence spectra are collected for samples with different ratio between the amount of porphyrins and quantum dots. Decay kinetic curves of CdSe quantum dots with and without porphyrins are well fitted by the biexponential decay curve, which indicates a combination of two components (excitonic and trapping state) in the luminescence behavior of CdSe quantum dots. Relative intensity weights of the excitonic and trapping state, total and nonradiative energy transfer efficiency, average luminescence lifetimes of donors are calculated. The nonradiative transfer mechanism becomes the leading factor as the concentration of acceptors gets higher. It is considered to take place through the channel of trapping states of CdSe quantum dots, which is expressed by the weight change between the fast and slow luminescence components. This deduction presents a new way of raising the energy transfer efficiency by increasing the trapping state proportion of the quantum dots, which can be easily realized by surface modification.

Nanoscale Confinement and Fluorescence Effects of Bacterial Light Harvesting Complex LH2 in Mesoporous Silicas

Many key chemical and biochemical reactions, particularly in living cells, take place in confined space at the mesoscopic scale. Toward understanding of physicochemical nature of biomacromolecules confined in nanoscale space, in this work we have elucidated fluorescence effects of a light harvesting complex LH2 in nanoscale chemical environments. Mesoporous silicas (SBA-15 family) with different shapes and pore sizes were synthesized and used to create nanoscale biomimetic environments for molecular confinement of LH2. A combination of UV–vis absorption, wide-field fluorescence microscopy, and in situ ellipsometry supports that the LH2 complexes are located inside the silica nanopores. Systematic fluorescence effects were observed and depend on degree of space confinement. In particular, the temperature dependence of the steady-state fluorescence spectra was analyzed in detail using condensed matter band shape theories. Systematic electronic-vibrational coupling differences in the LH2 transitions between ...

Probing the Hydrophobic Interactions of a Series of Pyrene End-Labeled Poly(ethylene oxide)s in Aqueous Solution Using Time-Resolved Fluorescence

The hydrophobic association of a series of poly(ethylene oxide)s covalently labeled at both ends with pyrene (PEO(X)-Py2 where X represents the number average molecular weight (Mn) of the PEO chains equal to 2, 5, 10, and 16.5 kDa) in aqueous solutions was investigated at different polymer concentrations (CP) using steady-state and time-resolved fluorescence measurements. Phase separation was observed with PEO(2 kDa)-Py2 and PEO(5 kDa)-Py2 samples at high CP. The steady-state fluorescence spectra showed that the ratios of excimer-to-monomer fluorescence intensities (IE/IM) of all PEO samples remained constant when CP was below 4 × 10(-5) M and decreased dramatically with increasing PEO chain length due to a decrease in intramolecular pyrene excimer formation. The IE/IM ratio in this regime was found to scale as Mn(-2.3±0.2). For CP > 4 × 10(-5) M, pyrene excimer is formed by both intra- and intermolecular interactions and the IE/IM ratio increases linearly with increasing CP except for PEO(2 kDa)-Py2 which undergoes phase separation. The decays obtained at various polymer concentrations were fitted according to a "sequential model" (SM) which assumes that the pyrene excimer is formed in a sequential manner. The molar fractions of all excited pyrene species and the rate constants for pyrene excimer formation were determined from the global analysis of the monomer and excimer fluorescence decays. The fraction of pyrenes that formed excimer from ground-state pyrene aggregates (fE0) was found to increase with CP in the regime where the pyrene excimer is formed both intra- and intermolecularly and decrease with Mn in the regime where the pyrene excimer is formed only intramolecularly. The fraction of pyrene pendants subject to hydrophobic interactions were used to determine the hydrophobic capture radius (Rc) of pyrene in water from the distribution of PEO end-to-end distances. Rc was found to equal 2.2 ± 0.2 nm using fE0.

Efficient collection of excitation energy from a linear-shaped weakly interacted perylenetetracarboxylic diimides array

A series of novel light-harvesting compounds (namely PO-PN, PO-PO-PN and PO-PO-PO-PN) were synthesized with a linear-shaped phenoxy group-substituted perylenetetracarboxylic diimide (PO) oligomer as donor and a pyrrolidinyl group-substituted perylenetetracarboxylic diimide (PN) as acceptor. The photophysical properties of these linear-shaped compounds are investigated by steady state electronic absorption, fluorescence spectra and lifetime measurements. The ground state interactions between the neighbor PO subunits within these three compounds are weak. No matter which PO subunit is excited in these linear molecules, the excitation energy is finally collected by the PN subunit. The excitation energy can transfer as long as 47 Å without any decrease in efficiency. The energy transfer rate constants determined by femtosecond transient absorption experiments are fast and close to that of the energy transfer from B800 to B850 in LH II of natural photosynthesis.

Electron transfer from CdSe–ZnS core–shell quantum dots to cobalt(iii) complexes

Fluorescence quenching of CdSe-ZnS quantum dots (core-shell QDs) is shown to be affected in the presence of cobalt(III) complexes with pyridyl anchors. Steady-state and time-resolved fluorescence spectra indicate that the quenching has primarily a static component. The decrease in photoluminescence intensity can be best explained by charge transfer from the QDs to the Co(III) complexes whereas the energy transfer pathways have been methodically ruled out. The fact that quenching is a result of electron transfer is also supported by electrochemical data showing the positions of the energy levels.

Modeling of Emission Spectra for Molecular Rings - LH2 And LH4 Complexes

Computer simulation of steady state fluorescence spectra of the ring molecular systems (resembling, e.g. the light harvesting rings from LH2 and LH4 photosynthetic complexes of purple bacteria) is presented in this paper. The general organization of the LH2 and LH4 complexes is the same: identical subunits are repeated cyclically in such a way that a ring-shaped structure is formed. However, the symmetries of these rings are different: LH2 is usually nonameric but LH4 is octameric. The other difference is the presence of four bacteriochlorophyll molecules per repeating unit in LH4 rather than three ones found in LH2. Transi- tion dipole moments of bacteriochlorophylls in B850 ring of LH2 have nearly tangential orientation whereas in LH4 they are organized in a more radial fashion. The dynamical aspects in ensemble of rings are reflected in optical line shapes of electronic transitions. The observed linewidths reflect the combined influence of different types of static and dynamic disorder. To avoid the broadening of lines due to ensemble averaging one uses the single-molecule spectroscopy technique to obtain a fluorescence-excitation spectrum. For our simulations we have used the ring of tightly bound two-level systems. Static disorder is taken into account simultaneously with dynamic disorder in Markovian approximation. The cumulant-expansion method of Mukamel et al. is used for the calculation of spectral responses of the system with exciton-phonon coupling. Comparison of steady state fluorescence spectra for B850 ring from LH2 and LH4 ring is done.

Solvent dependent photophysical properties of dimethoxy curcumin

Dimethoxy curcumin (DMC) is a methylated derivative of curcumin. In order to know the effect of ring substitution on photophysical properties of curcumin, steady state absorption and fluorescence spectra of DMC were recorded in organic solvents with different polarity and compared with those of curcumin. The absorption and fluorescence spectra of DMC, like curcumin, are strongly dependent on solvent polarity and the maxima of DMC showed red shift with increase in solvent polarity function (Δf), but the above effect is prominently observed in case of fluorescence maxima. From the dependence of Stokes’ shift on solvent polarity function the difference between the excited state and ground state dipole moment was estimated as 4.9D. Fluorescence quantum yield (ϕf) and fluorescence lifetime (τf) of DMC were also measured in different solvents at room temperature. The results indicated that with increasing solvent polarity, ϕf increased linearly, which has been accounted for the decrease in non-radiative rate by intersystem crossing (ISC) processes.

Preparation and time-resolved fluorescence study of RGB organic crystals

In this work, large-size tetracene- and pentacene-doped 1,4-Bis(4-methylstyryl)-benzene (BSB-Me) crystals were prepared with physical vapor transfer technique. The photoluminescence emission can be adjusted from blue to green and even to red by changing the doping. Their fluorescence features are investigated by both the steady-state fluorescence spectra and time-resolved fluorescence measurements, which indicate highly efficient energy transfer from the host BSB-Me to the acceptor. The PL efficiencies of the tetracene-doped BSB-Me and pentacene-doped BSB-Me crystals are as high as 79% and 47%, the energy transfer efficiency is 49.7% and 73.1%, respectively. Excitation intensity-dependent ultrafast dynamics was further probed, which shows that the kinetics in doped crystals exhibit much less excitation intensity dependence than that of the pure BSB-Me crystals. Our results are relevant to the understanding of ultrafast energy-transfer dynamics in doped crystals and are expected to be of interest for organic light emitting transistors, diodes, and lasers.

Nonlinear studies of malachite green polymer

Solid-state dye-doped polymer is an attractive alternative to the conventional liquid dye solution. In this paper the spectral characteristics and the nonlinear optical properties of the dye malachite green in polymethylmethacrylate is studied. The spectral characteristics are studied by recording steady state absorption and fluorescence spectra. Thermally induced nonlinearity of malachite green dye in n-butyl Acetate is studied using cw He–Ne laser at 632.8nm as source of excitation, both in solution and solid film. The property of dye in solid matrix is compared with that in liquid medium. The optical response is characterized by measuring the intensity dependent refractive index (n2) of the medium, using z-scan technique. The origin of optical nonlinearity in this dye may be attributed due to laser-heating induced nonlinear effect. The dye exhibited negative (defocusing) nonlinearity and it can be a promising material for optical limiting applications.

Fluorescence Response of Coumarin-153 in N-Alkyl-N-methylmorpholinium Ionic Liquids: Are These Media More Structured than the Imidazolium Ionic Liquids?

The fluorescence behavior of coumarin-153 (C153) has been studied in four N-alkyl-N-methylmorpholinium ionic liquids differing in the alkyl chain length attached to the N-methylmorpholinium cation as a function of the excitation wavelength and temperature to understand some of the physicochemical characteristics of these largely unexplored ionic liquids. While the polarity of the ionic liquid with the smallest alkyl chain length is found comparable to that of the commonly used imidazolium ionic liquids, the probe molecule experiences a less polar environment with increasing chain length of the alkyl group attached to the morpholinium cation. The room temperature steady-state fluorescence spectrum of C153 in these solvents is found to be dependent on the excitation wavelength, and this effect is most pronounced in long chain containing ionic liquids. A bathochromic shift of the fluorescence maximum is observed at higher temperature. The excitation wavelength and temperature dependence of the fluorescence of C153 is explained considering a domain structure of these ionic liquids. The time-resolved fluorescence anisotropy measurements indicate the microviscosity around the probe molecule to be significantly different from the bulk viscosity of the long-chain ionic liquids. The solvent reorganization dynamics, as studied by monitoring the time-dependent fluorescence Stokes shift of C153 in these ionic liquids, is found to be slow and similar to that in imidazolium ionic liquids. The time-resolved measurements under isoviscous conditions seem to provide additional support to the organized domain structure of these ionic liquids.

Solvent Effects on Spectral Property and Dipole Moment of the Lowest Excited State of Coumarin 343 Dye

Steady-state absorption and fluorescence spectra, and time-resolved fluorescence spectra of coumarin 343 (C343) were measured in different solvents. The effect of the solvent on the spectral properties and dipole moment of the lowest excited state of C343 were investigated. It was found that the absorption and fluorescence spectra red-shifted slightly and strongly with increasing solvent polarity, respectively, because the charge distribution of the excited state leaded to the increasing difference between the absorption and fluorescence spectra with increasing solvent polarity. The dipole moment of the lowest excited state of C343 was determined from solvatochromic measurements and the quantum chemical calculation, and the results obtained from these two methods were fully consistent. Investigations of the time-resolved fluorescence of C343 in different solvents indicated that the fluorescence lifetimes increased nearly linearly with increasing solvent polarity from 3.09 ns in toluene to 4.45 ns in water. This can be ascribed to the intermolecular hydrogen bonding interactions between C343 and hydrogen donating solvents.

The influences of central Ga atom and peripherally substituted F atoms on fluorescence properties of Corrole

The fluorescence properties of four kinds of corroles are investigated by steady-state and transient spectroscopy, which were influenced by the insertion of gallium (Ga) atom into Corrole rings and the increasing of the number of peripherally substituted fluorine atom (F). The steady-state absorption and fluorescence spectra show that the insertion of Ga atom can make the B and Q bands absorption of Corrole stronger while the increasing of peripherally substituented F atom weakens B and Q bands absorption. The insertion of Ga atom into corrole macrocyclic can greatly improve fluorescence quantum yield, and the same effect has been observed by increasing the number of the peripherally substituted F. The transient spectra show that the insertion of Ga atom can lead to shorter fluorescence lifetime, while the increasing of peripherally substituted F atom results in longer fluorescence lifetime. Therefore, the insertion of Ga atom into corrole macrocyclic can induce higher a 10 times larger radiative transition rate. On the other hand, the increasing of peripherally substituted F atom results in a slight increasing in radiative transition rate.

Transient excited-state absorption and gain spectroscopy of a two-photon absorbing probe with efficient superfluorescent properties.

The synthesis, linear photophysical properties, two-photon absorption (2PA), excited-state transient absorption, and gain spectroscopy of a new fluorene derivative tert-butyl 4,4'-(4,4' (1E,1'E)-2,2'-(9,9-bis(2- (2-ethoxyethoxy)ethyl)-9H-fluorene-2,7-diyl)bis(ethene-2,1-diyl)bis(4,1 phenylene)]dipiperazine-1-carboxylate (1) are reported. The steady-state linear absorption and fluorescence spectra, along with excitation anisotropy, fluorescence lifetimes, and photochemical stability of 1 were investigated in a number of organic solvents at room temperature. The 2PA spectra of 1 with a maximum cross-section of ~ 300 GM were obtained with a 1 kHz femtosecond laser system using open-aperture Z-scan and two-photon-induced fluorescence methods. The transient excited-state absorption (ESA) and gain kinetics of 1 were investigated by a femtosecond pump-probe methodology. Fast relaxation processes (~1-2 ps) in the gain and ESA spectra of 1 were revealed in ACN solution, attributable to symmetry-breaking effects in the first excited state. Efficient superfluorescence properties of 1 were observed in a nonpolar solvent under femtosecond excitation. One- and two-photon fluorescence microscopy imaging of HCT 116 cells incubated with probe 1 was accomplished, suggesting the potential of this new probe in two-photon fluorescence microscopy bioimaging.

Fluorescence spectral properties of stomach tissues with pathology

Steady-state fluorescence and diffuse reflection spectra are measured for in vivo normal and pathological (chronic atrophic and ulcerating defects, malignant neoplasms) stomach mucous lining tissues. The degree of distortion of the fluorescence spectra is estimated taking light scattering and absorption into account. A combination of Gauss and Lorentz functions is used to decompose the fluorescence spectra. Potential groups of fluorophores are determined and indices are introduced to characterize the dynamics of their contributions to the resultant spectra as pathologies develop. Reabsorption is found to quench the fluorescence of structural proteins by as much as a factor of 3, while scattering of the light can increase the fluorescence intensity of flavin and prophyrin groups by as much as a factor of 2.

Excitation energy transfer in covalently bonded porphyrin heterodimers

We describe the photophysical properties of heterodimers that are formed by the free base 2-(2-carboxyvinyl)-5,10,15,20-tetraphenylporphyrin and the zinc complex of 5-(p-aminophenyl)-10,15,20-triphenylporphyrin and that are covalently bonded by the amide link. These dimers differ in the configuration of the double bond in the spacer group. We determine fluorescence quantum yields of heterodimers and their porphyrin components. The energy transfer rate constants have been estimated from the measured fluorescence lifetimes and fluorescence excitation spectra and, also, they have been calculated from the steady-state absorption and fluorescence spectra according to the Forster theory. We have found that the efficiency of the intramolecular energy transfer in heterodimers is 0.97–0.99, and the energy migration rate constants have been found to be (1.82–4.49) × 1010 s−1. The results of our investigation show that synthesized heterodimers can be used as efficient light-harvesting elements in solar energy conversion devices.

Monomeric adenine decay dynamics influenced by the DNA environment

We report on-the-fly surface-hopping dynamics simulations of single adenine embedded in solvated DNA oligomers, (dA)10 and (dA)10·(dT)10. Both model systems are found to decay from the S1 to the S0 state via distinct monomeric channels, on account of the strong hydrogen-bonding interactions between the Watson-Crick pair in the double-stranded oligomer. Surprisingly, the decay times (several picoseconds) for the current models are 10 times longer than those of adenine in the gas or aqueous phase, while matching one of the time constants observed experimentally. We discuss possible reasons for these longer decay times, including steric hindrance in the DNA strands, electronic effects of the environment, and the presence of other local excited-state minima. We present optimized geometries and relative energies for representative S0 and S1 minima as well as conical intersections related to the hopping events. We have also computed steady-state and time-dependent fluorescence spectra that may help understand the experimental observations.

Artificially produced [7-formyl]-chlorophyll d functions as an antenna pigment in the photosystem II isolated from the chlorophyllide a oxygenase-expressing Acaryochloris marina

Acaryochloris marina, a chlorophyll (Chl) d-dominated cyanobacterium, is a model organism for studying photosynthesis driven by far-red light using Chl d. Furthermore, studies on A. marina may provide insights into understanding how the oxygenic photosynthetic organisms adapt after the acquisition of new Chl. To solve the reaction mechanism of its unique photosynthesis, photosystem (PS) II complexes were isolated from A. marina and analyzed. However, the lack of a molecular genetic method for A. marina prevented us from conducting further studies. We recently developed a transformation system for A. marina and we introduced a chlorophyllide a oxygenase gene into A. marina. The resultant transformant accumulated [7-formyl]-Chl d, which has never been found in nature. In the current study, we isolated PS II complexes that contained [7-formyl]-Chl d. The pigment composition of the [7-formyl]-Chl d-containing PS II complexes was 1.96±0.04 Chl a, 53.21±1.00 Chl d, and 5.48±0.33 [7-formyl]-Chl d per two pheophytin a molecules. In contrast, the composition of the control PS II complexes was 2.01±0.06 Chl a and 62.96±2.49 Chl d. The steady-state fluorescence and excitation spectra of the PS II complexes revealed that energy transfer occurred from [7-formyl]-Chl d to the major Chl d species; however, the electron transfer was not affected by the presence of [7-formyl]-Chl d. These findings demonstrate that artificially produced [7-formyl]-Chl d molecules that are incorporated into PS II replace part of the Chl d molecules and function as the antenna. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

In vivo measurements of diffuse reflectance and time‐resolved autofluorescence emission spectra of basal cell carcinomas

We present a clinical investigation of diffuse reflectance and time-resolved autofluorescence spectra of skin cancer with an emphasis on basal cell carcinoma. A total of 25 patients were measured using a compact steady-state diffuse reflectance/fluorescence spectrometer and a fibre-optic-coupled multispectral time-resolved spectrofluorometer. Measurements were performed in vivo prior to surgical excision of the investigated region. Singular value decomposition was used to reduce the dimensionality of steady state diffuse reflectance and fluorescence spectra. Linear discriminant analysis was then applied to the measurements of basal cell carcinomas (BCCs) and used to predict the tissue disease state with a leave-one-out methodology. This approach was able to correctly diagnose 87% of the BCCs. With 445 nm excitation a decrease in the spectrally averaged fluorescence lifetime was observed between normal tissue and BCC lesions with a mean value of 886 ps. Furthermore, the fluorescence lifetime for BCCs was lower than that of the surrounding healthy tissue in all cases and statistical analysis of the data revealed that this decrease was significant (p = 0.002).

Solvent Effect on the Fluorescence Spectra of Coumarin 120 in Water: A Combined Quantum Mechanical and Molecular Mechanical Study

The solvent effect on the steady-state and time-resolved fluorescence spectra of coumarin 120 in water was studied utilizing a molecular dynamics simulation with combined quantum mechanical/molecular mechanical method. The constructed steady-state fluorescence spectra reproduced the Stokes shift of the experimental data. The solvent effects on the spectra were examined by constructing three different spectra: spectra using the entire system, spectra including water molecules only in the first solvent shell, and spectra excluding all water molecules. We found that the variation in C–C bond length makes the largest contribution to the solvent shift in the fluorescence spectrum, which indicates the importance of the electronic structure variation.

Fluorescence properties of a composite material based on the 5CB nematic liquid crystal with gold nanoparticles

The steady-state and time-resolved fluorescence spectra of n-pentyl-n′-cyanobiphenyl and a related composite material with gold nanoparticles in the liquid crystalline and solid states have been investigated. The introduction of gold nanoparticles into the liquid crystal leads to a noticeable quenching of fluorescence in the emission region of the predimer and excimer states of cyanobiphenyl. In the solid phase, a quenching of free excitons and a short-wavelength shift of fluorescence bands have been observed.