Red-shifted Absorption Spectra Research Articles

Computations of absorption spectra and nonlinear optical properties of molecules based on anthocyanidin structure

Simulation of UV–VIS spectra and microscopic third-order nonlinear optical (NLO) coefficients at 532 nm wavelength of molecules based of anthocyanidin structure in gas phase environment using semi-empirical PM3 method was calculated. One-photon absorption characterizations of the investigated molecules have been theoretically obtained by configuration interaction method with doubly occupied molecular orbitals. Estimated theoretical spectra shows a sensibility to the different functional groups such as hydroxyl and methoxyl substitution in radicals. These groups causes red shift in absorption spectra in comparison to the other samples what agree well with the experimental data. Adjunction of such functional donor character group in the investigated molecule slightly decreases the HOMO–LUMO energy splitting gap but increases the ground state dipole moment resulting the substantial increase of NLO coefficient in this system comparing to the other samples. According to presented investigations the tested molecules have large nonlinear optical properties.

Unsymmetrical squaraine dye containing dithieno[3,2-b:2′,3′-d]pyrrole as a π-spacer: A potential photosensitizer for dye-sensitized solar cells

Designed with electron-rich and electron-deficient groups as π-linkers, a series of novel unsymmetrical squaraine dyes (SQ) for dye-sensitized solar cells (DSSCs) have been simulated using density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Dye/TiO2 complexes were also calculated to explore various properties on the interface. The results show that, compared with the prototype dye of WCH-SQ11, the modified WCH-SQ11-1, obtained by replacing the 3,4-ethylenedioxythiophene (EDOT) unit in WCH-SQ11 with dithieno[3,2-b:2′,3′-d]pyrrole (DTP), keeps a good balance in various key properties including light harvesting efficiency (LHE), driving force for electron injection (ΔGinject), charge transfer (CT) character, and vertical dipole moment (μnormal). Further investigation of the adsorbed WCH-SQ11-1 (i.e. the WCH-SQ11-1/(TiO2)38 complex) reveals a red-shifted absorption spectrum compared to the free dye. The analysis of the molecular orbitals responsible for the maximum absorption indicate a direct electron injection mechanism for the WCH-SQ11-1/(TiO2)38 system.

Simultaneous Effects of External Electric Field and Conduction Band Nonparabolicity on Optical Properties of a GaAs Quantum Dot Embedded at the Center of a GaAlAs Nano-Wire

An investigation of the optical properties of a GaAs spherical quantum dot which is located at the center of a Ga1−xAlxAs cylindrical nano-wire has been performed in the presence of an external electric held. The band nonparabolicity effect is also considered using the energy dependent effective mass approximation. The energy eigenvalues and corresponding wave functions are calculated by finite difference approximation and the reliability of calculated wave functions is checked by computing orthogonality. Using computed energy eigenvalues and wave functions, the linear, third-order nonlinear and total optical absorption coefficients and refractive index changes are examined in detail. It is found that (i) Presence of electric field causes both blue and red shifts in absorption spectrum; (ii) The absorption coefficients shift toward lower energies by taking into account the conduction band nonparabolicity; (iii) For large values of electric field the effect of conduction band nonparabolicity is less dominant and parabolic band is estimated correctly; (iv) In the presence of electric field and conduction band nonparabolicity the nonlinear term of absorption coefficient rapidly increases by increasing incident optical intensity. In other words, the saturation in optical spectrum occurs at lower incident optical intensities.

A simple levulinate-based ratiometric fluorescent probe for sulfite with a large emission shift.

A simple 4-hydroxynaphthalimide-derived colorimetric and ratiometric fluorescent probe (1) containing a receptor of levulinate moiety was designed and synthesized to monitor sulfite. Probe 1 could quantificationally detect sulfite by a ratiometric fluorescence spectroscopy method with high selectivity and sensitivity. Specially, probe 1 exhibited a 100 nm red-shifted absorption spectrum along with the color changes from colorless to yellow, and 103 nm red-shifted emission spectra upon the addition of sulfite. Thus, 1 can serve as a "naked-eye" probe for sulfite. Further, the recognition mechanism of probe 1 for sulfite was confirmed using nuclear magnetic resonance and electrospray ionization mass spectrometry. Also, the preliminary practical application demonstrated that our proposed probe provided a promising method for the determination of sulfite.

Computational study on zinc porphyrin analogs for use in dye-sensitized solar cells

The density functional theory (DFT) and time-dependent DFT (TD-DFT) approaches have been applied to obtain the optimized geometries, electronic structures, molecular orbitals and absorption spectra of a series of meso-substituted zinc porphyrin analogs with phenyl and thiophene groups as the π bridging unit and cyanoacrylic acid as the acceptor unit. The results showed that the introduction of thiophene group increases the orbital splitting and changes the absorption spectra properties significantly. It is indicated that when there is only one thiophene group included in the π bridge, the oscillator strength of B absorption band is much stronger. The increasing length of thiophene chain just changes the scope of specific absorption enhancement. The effect of attaching an additional electron-donating group diphenylamine instead of phenyl to the porphyrin core also has been shown. It is found that the diphenylamine group reduces the band gap, and leads to facile intramolecular charge transfer from diphenylamine and porphyrin ring unit to acceptor unit. These kinds of zinc porphyrin analogs have the LUMO energy close to the conduction band of TiO 2 and more red-shifted absorption spectrum compared with phenyl substituted analogs.

Effect of electron withdrawing unit for dye-sensitized solar cell based on D-A-π-A organic dyes

In this work, two novel D-A-π-A dye sensitizers with triarylamine as an electron donor, isoindigo and cyano group as electron withdrawing units and cyanoacetic acid and 2-(1,1-dicyanomethylene) rhodanine as an electron acceptor for an anchoring group (TICC, TICR) were designed and investigated with the ID6 dye as the reference. The difference in HOMO and LUMO levels were compared according to the presence or absence of isoindigo in ID6 (TC and ID6). To gain insight into the factors responsible for photovoltaic performance, we used density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. Owing to different LUMO levels for each acceptor, the absorption band and molar extinction coefficient of each dye was different. Among the dyes, TICR showed more red-shifted and broader absorption spectra than other dyes and had a higher molar extinction coefficient than the reference. It is expected that TICR would show better photovoltaic properties than the other dyes, including the reference dye.

Self-assembled nanostructured photoanodes with staggered bandgap for efficient solar energy conversion.

Vertically oriented Ta-W-O nanotube array films were fabricated via the anodization of Ta-W alloy foils in HF-containing electrolytes. HF concentration is a key parameter in achieving well-adhered nanotube array structure. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance measurements confirm the staggered band-alignment between Ta2O5 and WO3, which facilitates the separation of charge carriers. The nanotubes made of Ta-W films containing 10% W showed 100-fold improvement in the measured photocurrent compared to pristine Ta2O5 upon their use to split water photoelectrochemically. This enhancement was related to the efficient charge transport and the red shift in absorption spectrum with increase of the W content, which was asserted by ultrafast transient absorption (TA) spectroscopy measurements. The TA measurements showed the elimination of trap states upon annealing Ta-W-O nanotubes and, hence, minimizing the charge carrier trapping, whereas the trap states remain in pristine Ta2O5 nanotubes even after annealing.

A fast-response, highly sensitive and selective fluorescent probe for the ratiometric imaging of hydrogen peroxide with a 100 nm red-shifted emission

Recently, growing attention has been paid to the accurate determination of hydrogen peroxide (H2O2) for elucidating its detailed biological function in physiology and pathology. A fluorescence method with the help of a fluorescent probe is the preferred technique for in situ visualization of biologically important species in vivo, even in single living cells. In the present manuscript, we developed a simple, fast response and highly selective fluorescent probe (1) with a receptor of the boronate moiety for the ratiometric imaging of H2O2 in living cells. Probe 1 could quantifiably detect H2O2 in the range of 18–540 μM by a ratiometric fluorescence spectroscopy method with a detection limit of 4 μM. Importantly, probe 1 exhibited 81 nm red-shifted absorption spectra accompanied by the color changes from colorless to yellow, and 100 nm red-shifted emission spectra upon addition of H2O2. Thus, 1 can serve as a “naked-eye” probe for H2O2. Preliminary bioimaging application and low cytotoxicity investigations further demonstrated that the proposed probe would be of great benefit to biomedical researchers for investigating the detailed biological function of H2O2 in biological systems.

A resorufin-based colorimetric and fluorescent probe for live-cell monitoring of hydrazine

We report a novel colorimetric and red-emitting fluorescent probe for hydrazine detection based on resorufin platform. This OFF–ON fluorescent probe shows a large (117nm) red-shifted absorption spectrum and the color changes from colorless to red upon addition of hydrazine in the aqueous solution, which can serve as a “naked-eye” probe for hydrazine. Moreover, this probe also shows a significant fluorescence increase (~16 folds) and excellent linear relationship at physiological pH. Utilizing this sensitive and selective probe, we have successfully detected hydrazine in living cells.

First-principles study of Carbz-PAHTDDT dye sensitizer and two Carbz-derived dyes for dye sensitized solar cells

Two new carbazole-based organic dye sensitizers are designed and investigated in silico. These dyes are designed through chemical modifications of the π-conjugated bridge of a reference organic sensitizer known as Carbz-PAHTDDT (S9) dye. The aim of designing these dyes was to reduce the energy gap between their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and to red-shift their absorption response compared to those of the reference S9 dye sensitizer. This reference dye has a reported promising efficiency when coupled with ferrocene-based electrolyte composition. To investigate geometric and electronic structure, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were conducted on the new dyes as well as the reference dye. The present study indicated that the long-range correction to the theoretical model in the TD-DFT simulation is important to produce accurate absorption wavelengths. The theoretical studies have shown a reduced HOMO-LUMO gap and red-shifted absorption spectra for both of the new candidate dyes. In particular, the new S9-D1 dye is found to have significant reduced HOMO-LUMO energy gap, greater push-pull character and higher wavelengths of absorption when compared to the reference dye. Such findings suggest that the new dyes are promising and suitable for optoelectronic applications.

Facile colorimetric detection of Hg2+ based on anti-aggregation of silver nanoparticles

This paper describes an investigation of a facile colorimetric sensor for Hg2+ in aqueous solution based on the anti-aggregation of silver nanoparticles (AgNPs). In the absence of Hg2+, the addition of 6-Thioguanine to AgNPs solution led to the aggregation of AgNPs, resulting in a color change from yellow to brown with a red shift of absorption spectra. However, the presence of Hg2+ inhibited the 6-Thioguanine-induced aggregation of AgNPs accompanying with a color change from brown to yellow. Under the optimal conditions, the ratio between the absorbance at 530nm and 394nm (A530/A394) was linearly proportional to the Hg2+ concentration in a range from 0 to 333nM with a detection limit of 4nM. Other environmentally relevant metal ions did not interfere with the detection of Hg2+. The proposed method was simple, cost-effective and rapid without any complicated modifying step. It was successfully applied to detect Hg2+ in environmental water samples.

Effect of zinc oxide concentration in fluorescent ZnS:Mn/ZnO core–shell nanostructures

In the present work, we have prepared zinc sulphide (ZnS:Mn)/zinc oxide (ZnO) core–shell nanostructures by a chemical precipitation method and observed the effect of ZnO concentration on the fluorescent nanoparticles. Change in the morphological and optical properties of core–shell nanoparticles have been observed by changing the concentration of ZnO in a core–shell combination with optimum value of Mn to be 1 % in ZnS. The morphological studies have been carried out using X-ray diffraction (XRD) and transmission electron microscopy. It was found that diameter of ZnS:Mn nanoparticles was around 4–7 nm, each containing primary crystallites of size 2.4 nm which was estimated from the XRD patterns. The particle size increases with the increase in ZnO concentration leading to the well-known ZnO wurtzite phase which was coated on the FCC phase of ZnS:Mn. Band gap studies were performed by UV–visible spectroscopy and a red shift in absorption spectra have been observed with the addition of Mn as well as with the capping of ZnO on ZnS:Mn. The formation of core–shell nanostructures have been also confirmed by FTIR analysis. Photoluminescence studies show that emission wavelength is red shifted with the addition of ZnO layer on ZnS:Mn(1 %). These core–shell ZnS:Mn/ZnO nano-composites will be a very suitable material for specific kind of tunable optoelectronic devices.

Strong Photophysical Similarities between Conjugated Polymers and J-aggregates.

The photophysical properties of emissive conjugated polymer (CP) chains are compared to those of linear J-aggregates. The two systems share many properties in common, including a red-shifted absorption spectrum with increasing chain/aggregate length, enhanced radiative decay rates (superradiance) relative to a single monomer/molecule, and several vibronic signatures involving the vinyl-stretching mode common to many conjugated molecules. In particular, the scaling of the 0-0/0-1 photoluminescence ratio and radiative decay rate with the inverse square root of temperature in red-phase polydiacetylene is also characteristic of linear, disorder-free J-aggregates. The strong photophysical resemblance is traced to the excitonic band structure; in one-dimensional direct band gap semiconductors as well as J-aggregates, the exciton band curvature is positive at the gamma point (k = 0).

光致电子转移对Alizarin敏化TiO2纳米粒子Raman光谱特性的影响

Effects of photoinduced electron transfer on the Raman spectroscopy of Alizarin(Alz)dyesensitized TiO2nanoparticles were investigated by theoretical and experitmental methods.The results indicate that red-shift of absorption spectra and quenching of the fluorescence spectra are attributed to the photo-induced electron transfers from the excited state of the absorbed Alz dye molecules and the charge transfer complex(Alz/TiO2)to the conduction band manifold of the TiO2nanoparticles.The resonance Raman spectroscopy of the Alz dye-sensitized TiO2nanoparticles reveals that the ultrafast photoinduced interface electron transfer significantly enhances the C=C,C-O,and C=O bond stretching vibrations of the absorbed Alz dye molecule at the interface.

First-principle study on the effect of high Pr doping on the optical band gap and absorption spectra of TiO2

Nowadays, the studies on band gap and absorption spectrum of TiO2 doped with Pr lead to opposite conclusions. Two experimental results about red-shift and blue-shift are reported in the literature. We have set up models for pure TiO2 and different doping concentrations of Pr-doped TiO2 to calculate the electronic structure and absorption spectrum based on the first-principle plane-wave ultrasoft pseudopotential in terms of the density functional theory (DFT) to slove the above problem. Results indicate that under the condition of heavy doping Pr, compared with the pure TiO2, as the Pr concentration increases the atomic charge of the doped system reduces and the total energy of the doped system becomes higher, and its formation energy will be greater. This makes the stability decline, the band gap narrowed, the absorption spectrum red-shift, and the absorption strength more significant. The results of the calculation is in agreement with the experimental data.

The effect of five-membered heterocyclic bridges and ethoxyphenyl substitution on the performance of phenoxazine-based dye-sensitized solar cells

Phenoxazine derivatives with heterocyclic bridge units and an additional donor were synthesized and applied to dye-sensitized solar cells. To study the effects of these substituents on the DSSC performance, the photophysical, electrochemical, and photovoltaic properties of the dyes were investigated. The introduced heterocyclic bridge units furan and thiophene improved the short-circuit current due to the red-shifted absorption spectra of the dyes. The ethoxyphenyl ring introduced to the phenoxazine moiety as an additional donor broadened the spectrum of the dye, while the reduced adsorption of the dye caused by its non-planar structure limited the enhancement of the short-circuit current. Among the synthesized dyes, the one with furan as a bridge unit showed the best overall conversion efficiency of 5.26%.

ZnO and Co-ZnO nanorods—Complementary role of oxygen vacancy in photocatalytic activity of under UV and visible radiation flux

Abstract Enhancement in photoactivity of zinc oxide, a UV active photocatalyst, can be achieved through nanostructuration in addition to doping induced red-shift in absorption spectrum. For this, nanorods of cobalt-doped-ZnO (Co-ZnO) were synthesized by hydrothermal method. As expected, Co-ZnO nano-rods showed high photocatalytic activity in degradation of methylene blue dye (MB) and phenol under visible radiation flux compared to pristine ZnO nano-rods, synthesized identically. XRD, SEM, TEM, EDX, UV-DRS, PL and FTIR studies were used to characterize the photocatalysts. ZnO and Co-ZnO, both had wurtzite phase of ZnO with nano-rod morphology having length:diameter of 440 nm:120 nm and 1460 nm:162 nm, respectively. Surprisingly, under UV light the ZnO-nanorods showed more activity than Co-ZnO. These contrasting results, when correlated with dopant-induced quenching of oxygen vacancies, indicated toward complementary role of oxygen vacancy in the photoactivity of the materials with the dominant role of band gap-assisted red-shift in the absorption spectrum.

Near-Infrared Colorimetric and Fluorescent Cu2+ Sensors Based on Indoline–Benzothiadiazole Derivatives via Formation of Radical Cations

The donor-acceptor system of indoline-benzothiadiazole is established as the novel and reactive platform for generating amine radical cations with the interaction of Cu(2+), which has been successfully exploited as the building block to be highly sensitive and selective near infrared (NIR) colorimetric and fluorescent Cu(2+) sensors. Upon the addition of Cu(2+), an instantaneous red shift of absorption spectra as well as the quenched NIR fluorescence of the substrates is observed. The feasibility and validity of the radical cation generation are confirmed by cyclic voltammetry and electron paramagnetic resonance spectra. Moreover, the introduction of an aldehyde group extends the electron spin density and changes the charge distribution. Our system demonstrates the large scope and diversity in terms of activation mechanism, response time, and property control in the design of Cu(2+) sensors.

Intermolecular Interaction as the Origin of Red Shifts in Absorption Spectra of Zinc-Phthalocyanine from First-Principles

We investigate electronic origins of a redshift in absorption spectra of a dimerized zinc phthalocyanine molecule (ZnPc) by means of hybrid density functional theoretical calculations. In terms of the molecular orbital (MO) picture, the dimerization splits energy levels of frontier MOs such as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the constituent molecules. Consequently, the absorption wavelength seems to become longer than the monomer as the overlap between the monomers becomes larger. However, for a ZnPc dimer configuration with its cofacially stacked monomer arrangement, the calculated absorption spectra within the time-dependent density functional theory indicates no redshift but blueshift in the Q-band absorption spectrum, i.e., a typical H-aggregate. The origin of the apparently contradictory result is elucidated by the conventional description of the interaction between monomer transition dipoles in molecular dimers [Kasha, M. Radiat. Res. 1963, 20, 55]. The redshift is caused by an interaction between the two head-to-tail transition dipoles of the monomers, while the side-by-side arranged transition dipoles result in a blueshift. By tuning the dipole-dipole interaction based on the electronic natures of the HOMO and the LUMO, we describe a slipped-stacked ZnPc dimer configuration in which the Q-band absorption wavelength increases by as large as 144 nm relative to the monomer Q-band.

A highly selective colorimetric and long-wavelength fluorescent probe for Hg2+

A simple cell-permeable long-wavelength fluorophore 1-(3-cyano-2-dicyanomethylen-5,5-dimethyl-2,5-dihydrofuran-4-yl)-2-(4-hydroxylphenyl)ethane (2) was designed and synthesized. Based on this novel fluorophore, probe 1 employing a vinyl group as a recognition receptor was synthesized to highly selective detect Hg2+ in the presence of other metal ions. Probe 1 could quantificationally detect Hg2+ by the absorption and fluorescence spectroscopy methods with excellent sensitivity. Additionally, our proposed probe exhibited a large (162nm) red-shifted absorption spectrum and the color changes from yellow to blue upon addition of Hg2+. The reaction mechanism of probe 1 and Hg2+ was confirmed using 1H-NMR, 13C-NMR, and ESI-MS. Practical application and cell imaging of probe 1 further demonstrated that it provided an excellent method for the determination of Hg2+ in the environment and living systems.