Red-shifted Emission Band Research Articles

Synthesis, crystal structure and spectroscopic properties of an unsymmetrical compound with carbazole and benzothiadiazole units

An unsymmetrical organic compound with carbazole (Cz) as donor and benzothiadiazole (BTD) as acceptor (D-π-A-π∗-D∗) was designed and synthesized via simple Heck reaction. The unique crystal structure of Cz–BTD–Cz∗ shows a ladder-like packing mode. A two molecule pair stacks parallelly with each other in each packing unit. In each cell, one Cz moiety is connected with BTD by vinylene bond in same plane. However, the other Cz group is connected to BTD by a one-end vinyl bond in almost perpendicular position to the coplanar part of the molecule. The shortest intermolecular plane distance is 3.48 ± 0.1 Å. The photophysical properties of Cz–BTD–Cz∗ in solution and in bulky crystalline powder state were studied. In bulk crystalline powder state, it has a red-shifted emission band peaked at 609 nm relative to that in solution, and the FWHM was reduced to only 58 nm. Electrochemical properties were also investigated.

Effect of surfactants on the fluorescence spectra of water-soluble MEHPPV derivatives having grafted polyelectrolyte chains

Poly(2-methoxy-5-[2′-ethylhexyoxy]-1,4-phenylenevinylene) (MEHPPV) derivatives with polyacrylic acid (PAA) chains grafted onto their backbone were found to be water soluble, and they exhibited a dramatic increase in their fluorescence intensity in the presence of a variety of surfactants, even at concentrations far below their critical micelle concentrations (CMC). This increase was accompanied by a blue-shift in the emission maximum. These observations are rationalized based on the postulate that the backbone conformation of the conjugated polymer is modulated upon interaction of the surfactant molecules with the polyelectrolytic tethers, which in turn results in a significant depletion of intra-chain interchromophore interactions that are known to cause red-shifted emission bands with significantly lower emission yields.

Photo-physical properties of 1-hydroxy-2-naphthaldehyde: A combined fluorescence spectroscopy and quantum chemical calculations

The ground and excited state properties of 1-hydroxy-2-naphthaldehyde (HN12) towards proton transfer reaction have been elaborately investigated on the basis of steady state absorption and emission, time-resolved fluorescence spectroscopy and quantum chemical calculations by ab initio (Hartree–Fock) and density functional theory (DFT) methods. The molecule HN12 exists as closed (intramolecular hydrogen bonded) and open conformer and their anions, and the hydrogen bonded solvated cluster in the ground state. The closed conformer on excitation undergoes photo-induced keto-enol tautomerism across the pre-existing intramolecular hydrogen bond (IMHB) and shows a red shifted emission band for the keto tautomer. Theoretically a significant change of some structural parameters such as O d–H 1 bond length, O d⋯H 1⋯O a bond angle and charge distribution at the proton translocation site in the excited state favours intramolecular proton transfer process. Calculated potential energy curves along the proton transfer coordinate at the pre-existing hydrogen bonding site by DFT level account well for the feasibility of a barrierless ultrafast excited state intramolecular proton transfer reaction in HN12 molecule.

Luminescence properties of dual fluorescent pyrene derivatives used as DNA probe molecule

We examined the dual fluorescence mechanism of 8-[4-(dimethylamino)phenyl]-N-2-propynyl-1-pyrenecarbox-amide (PyADMA) in polar solvent. The emission band appeared on the short wavelength side does not depend on the solvent polarity. On the other hand, the red shifted emission band depends on solvent polarity. On the face of it, the phenomenon is similar to intramolecular charge transfer (ICT) dynamics of 4-dimethylamino benzonitrile (DMABN). However, direct absorption band from ground state to ICT state is observed on the fluorescence excitation spectrum at room temperature. It is suggest that PyADMA would emit from both S(1) and S(2) state against the Kasha rule. On the other hand, we also examined the interaction between PyADMA and nucleoside which composing PyDMA-U in water solution. We found out that dynamics of excited states of PyADMA monomer is different from them of PyDMA-U.

Secondary amino group as charge donor for the excited state intramolecular charge transfer reaction in trans-3-(4-monomethylamino-phenyl)-acrylic acid: Spectroscopic measurement and theoretical calculations

The emission properties of a synthesized donor–acceptor substituted aromatic system trans-3-(4-monomethylamino-phenyl)-acrylic acid ( t-MMPAA) have been investigated both experimentally and theoretically. The molecule t-MMPAA shows dual fluorescence in polar solvents. The normal emission band is assigned to be arising from the locally excited state whereas the anomalous, solvent dependent, red shifted emission band arises from a twisted transient emissive state. Solvent dependency of the large Stokes shifted emission band and high dipole moment of the excited state support charge transfer character of the twisted excited state. Potential energy curves along the two possible twisting coordinates have been calculated for the ground and excited states both in vacuo and in polar aprotic acetonitrile solvent using time dependent density functional theory (TDDFT) and time dependent density functional theory including polarized continuum model (TDDFT-PCM), respectively. Both the twisted paths, i.e., along the donor (–NHMe) and acceptor (–CH CHCOOH) groups, favor the formation of stable twisted excited state, but the twisting through acceptor path is energetically favorable (low barrier height) than the donor path. However, localized nitrogen lone pair supports TICT process by twisting along the donor path. A more stabilized calculated twisted state in acetonitrile solvent correlates well with the solvent dependent red shifted emission.

Spectroscopic studies of liquid solutions of R6G laser dye and Ag nanoparticle aggregates

We have found that R6G laser dye in a concentration of 0.1 g l−1 mixed with a solution of aggregated silver nanoparticles exhibits a new emission band with a maximum at 612 nm. This band does not exist in pure dye of comparable concentration or in a mixture of dye with a solution of single silver nanoparticles. A qualitatively similar red-shifted emission band is observed in pure R6G dye at very high concentration (3.8 or 16.7 g l−1). In both cases, no changes occur to the shapes of the absorption spectra of the dye. We explain the observed spectral changes in terms of J-aggregates of R6G molecules whose formation is probable in the presence of Ag aggregates with a complicated surface structure and is much less likely in the case of adsorption of dye molecules on single Ag nanoparticles. Alternatively, many features observed in the experiment can be explained by an enhancement of the rates of spontaneous radiative transitions in the proximity of metallic particles, which is due to a modification of the local density of electromagnetic modes in the vicinity of metal surfaces at energies resonant with surface plasmon resonances.

7‐(Dimethylamino)tricyclo[5.2.2.01,6]undecene Derivatives from β‐Cyclohexenyl β‐Dimethylamino‐Substituted α,β‐Unsaturated Fischer Carbenes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Substitution of a Chlorophyll into the Inactive Branch Pheophytin-Binding Site Impairs Charge Separation in Photosystem II

All photosynthetic reaction centers (RCs) have two parallel sets of electron transfer cofactors that cross the membrane. In quinone-type RCs (including photosystem II (PSII)), however, only one pathway (the active branch) is used for electron transfer. Since the electron transfer cofactors of each pathway have nearly identical distance and orientation relationships, it is assumed that local differences in protein environment determine the directionality of electron transfer. To understand further the factors that affect energy distribution among the PSII RC cofactors, we altered the PSII RC cofactor symmetry by replacing the inactive-branch pheophytin (Pheo) with a chlorophyll (Chl). We mutated the D1-L210 residue to a histidine (D1-L210H) to provide a Mg ligand for Chl. Analyses of the pigment composition of D1-L210H RCs indicated that the inactive-branch Pheo had been replaced by a Chl. Comparisons of wild-type and D1-L210 transient absorption spectra confirmed that the red-shifted Pheo Qx absorption band (543.5 nm) belonged to the active-branch Pheo. Surprisingly, intact D1-L210H PSII complexes were unable to evolve oxygen, lacked Chl variable fluorescence, (following a flash), and were unable to photoaccumulate reduced Qa, indicating that electron transfer in D1-L210H PSII complexes was severely perturbed. The kinetics of primary charge separation, however, were not substantially altered in D1-L210H RCs, indicating that the Chl substitution had not perturbed the energetics of the primary electron donor/acceptor pair. Significantly, intact D1-L210H PSII core complexes had a substantially increased and red-shifted Chl fluorescence emission band attributed to fluorescence from Chl's of the distal antenna complex as well as a blue-shifted fluorescence emission peak attributed to Chl's of the proximal antenna complex (77 K). These results are interpreted in terms of a redistribution of the excited-state energy among the pigments of the RC multimer, leading to loss of the excited state via fluorescence in the D1-L210H mutant.

Conformationally regulated fluorescent sensors. Study of the selectivity in Zn2+ versus Cd2+ sensing

The Zn2+ and Cd2+ complexing properties of four ligands containing a 4,4′-substituted biphenyl moiety are described. Ligands 1 and 3, containing only one 1-aza-18-crown-6 cavity, lead to selective complexation of Cd2+ versus Zn2+. Ligand 4, with two crown cavities linked to a tetramethylbenzidine unit, is able to form 1:1 complexes with Zn2+ and Cd2+, showing a higher complexing constant with Zn2+ than with Cd2+, probably due to enthalpic factors. Several complementary experiments suggest that the 1:1 complexes formed by ligand 4 involve both crown cavities acting together to give rise to clamp structures. The formation of this type of zinc complex gives rise to red shifted emission bands and distinct quenching of the fluorescence. A similar situation is observed with cadmium but the change is then less pronounced. When mixtures of both salts are used, ligand 4 selectively responds to zinc. Finally, ligand 2, which also has two crown cavities but contains nitro rather than amino groups in the biphenyl moiety, shows no propensity to form clamp complexes and, for this reason, it complexes cadmium much more strongly than zinc and binds the former selectively when mixtures of both salts are used in complexing experiments.

Fluorescent Crystallization of Isomeric Quinoline-Capped Thiophene Oligomer in Thin Films Grown by Hot-Wall Epitaxy

Crystal growth and fluorescent characteristics of a quinoline-capped thiophene oligomer were investigated in thin films grown on the (001) surface of KCl by a hot-wall epitaxy technique. Under controlled substrate and wall temperatures, the deposited film consisted of two types of crystallites: one was epitaxial needles oriented along the 〈100〉KCl and 〈110〉KCl directions, and another was irregular aggregates among them. Fluorescence spectromicroscopy revealed that the former crystals exhibited a red-shifted emission band with respect to the latter. Microscopic Raman spectra for the individual crystallites and vibrational analysis by molecular orbital calculations identified the origin of this fluorescence difference as cis−trans isomerization of the peripheral quinoline groups. In the epitaxial needles, the trans-form molecules were lying parallel to the KCl surface while in the irregular aggregates the cis-form molecules were standing. The detailed crystallization depending on the isomerization was also supported by scanning near-field/confocal scanning laser and transmission electron microscopies.

Fluorescence of crystalline 4-(dimethylamino)benzonitrile. Absence of dual fluorescence and observation of single-exponential fluorescence decays

The fluorescence spectrum of crystals grown from newly synthesized 4-(dimethylamino)benzonitrile (DMABN), measured from 25 down to −112 °C, consists of a single emission band originating from a locally excited (LE) state. The fluorescence decay of the DMABN crystals is single exponential at all temperatures investigated. These results show that intramolecular charge transfer (ICT) does not occur in crystalline DMABN. The additional red-shifted emission bands and multiexponential fluorescence decays previously reported for DMABN crystals are attributed to a minor amount of the impurity 4-(dimethylamino)benzaldehyde, the synthetic precursor of commercial DMABN.

Spectral and Photophysical Properties of Fluorene-Based Polyesters in Solution and in the Solid State

The optical and photophysical properties of fluorene-based polyesters in solution and in the solid state (thin films) are reported. The red-shifted emission bands observed in the photoluminescence spectra of the thin films are rationalized in terms of stronger π−π intermolecular interactions due to the closeness of the molecules (molecular packing) in the solid state. This is further corroborated by time-resolved fluorescence decay measurements. However, these electronic interactions are not strong enough to induce excimer or aggregate formation. As such, the quenching excimer and/or aggregate deactivation channel found in many polymeric systems are less efficient in the present polyesters. As expected, the presence of ester linkages between adjacent oligomers causes an interruption of the electronic conjugation, thus limiting the fluorescence to the blue region. The fluorescence efficiencies of thin films, estimated from lifetime measurements of the polyesters in solution and in the solid state, are relatively high (10−25%). Hence, these materials are promising candidates in the fabrication of blue-emitting LED devices.

Spectroscopic Study of Intermolecular Interactions in Various Oligofluorenes: Precursors of Light-Emitting Polymers

A detailed analysis of the optical and photophysical properties of 2,7-bis(phenylene)-9,9-dioctylfluorene (PFP), 2,7-bis(biphenylene)-9,9-dioctylfluorene (BPFBP), 2,7-bis(2-thienyl)-9,9-dioctylfluorene (TFT), and 2,7-bis-(2,2'-bithien-5-yl)-9,9-dioctylfluorene (BTFBT) in various environments are reported. The optical properties of the free molecules isolated in an alkane matrix are obtained and discussed in terms of the conformation adopted by each derivative in the electronic ground and first excited states. Also, conformational changes are responsible for the optical changes observed at high concentrations in an isopentane glass at 77 K. High quantum yields of all the oligofluorenes at 77 K indicate the absence of quenching effects such as excitonic or aggregation effects. The similar spectral and photophysical properties in matrix and glass environments are explained by the disorder introduced in oligofluorenes by long octyl chains at the C-9 position of the fluorene moiety. To study the effect of intermolecular interactions in the solid state, we recorded the spectra of thin films of these derivatives. The much red-shifted emission band in the solid state cannot be explained by conformational changes and has its origin in the π-stacking of conjugated oligomers in their relaxed S 1 state. As an evidence to show the importance of the role played by octyl chains at the C-9 position of the fluorene moiety, we synthesized two new model compounds: one, without octyl chains at the C-9 position of the fluorene moiety, 2,7-bis(2-thienyl)fluorene (TFTWC) and another with more octyl chains, 1,4-bis(9,9-dioctylfluoren-2-yl)phenyl (FPF). The spectral properties of these derivatives have been studied at room temperature and at 77 K. These systems serve as excellent examples to show the effect of intermolecular interactions on optical properties of oligofluorenes.

Impact of In Bulk and Surface Segregation on the Optical Properties of (In,Ga)N/GaN Multiple Quantum Wells

We study the optical properties of (In,Ga)N/GaN multiple quantum wells grown by plasma-assisted molecular-beam epitaxy (MBE). N-stable growth inevitably delivers rough interfaces, resulting in strongly red-shifted and broad emission bands. For metal-stable growth, secondary ion-mass spectrometry gives evidence of quantum wells with smooth interfaces but significantly larger width than intended, indicating a strong surface segregation of In. In addition, these measurements reveal that the redistribution of In leads to top-hat In profiles with abrupt leading and trailing edges. The strongly blue-shifted transition energy for these quantum wells may be the reason for the frequent conclusion that the theoretical polarization fields of Bernardini et al. [Phys. Rev. B 56, R10024 (1997)] are too large for (In,Ga)N. Being in possession of the (at least approximately) correct structural parameters, we find the theoretical fields to be in very satisfactory agreement with those deduced from experimental data. A further consequence of the In surface segregation is a pronounced reduction in the quantum efficiency due to a reduced electron-hole overlap. (In,Ga)N is additionally expected to be subject to In bulk segregation, leading to a localization of excitons at compositional fluctuations. Spot-excitation cathodoluminescence spectroscopy directly reveals the presence of localized states. For a quantitative understanding we investigate the temperature dependence of the radiative decay time which is measured by time-resolved photoluminescence spectroscopy and utilized as a probe of the dimensionality of the system. The analysis of these experiments within the frame of a coupled rate-equation model yields a localization depth in these MBE-grown (In,Ga)N/GaN quantum wells of around 20-30 meV. Even this comparatively shallow localization is found to significantly enhance the internal quantum efficiency up to a temperature of about 100 K.

Fluorescent phthalocyanine dimers—a steady state and flash photolysis study

Steady state and time resolved studies of a tetrakis[(+/-)-2,2-dimethyl-1,3-dioxan-4-ylmethanol] (solketal) substituted zinc phthalocyanine (1) and its derivatives (4a-c) are reported. These compounds form novel fluorescent dimers (or higher order aggregates) at low temperature, with a red shifted emission band centred at approximately 750 nm. The degree of aggregation has been related to the nature of the peripheral substituents, with bulky groups reducing and long alkyl chains increasing the phenomenon. Fluorescence lifetimes of the dimeric species are reported. Flash photolysis results show complex behaviour of the transient species, and it is possible to identify three main processes, involving decay of the monomer triplet state, the dimer triplet, and dimer disaggregation, with each process having a characteristic transient lifetime.

Fluorescence of a ketocyanine dye in pure and mixed binary solvents at 77 K

Emission and excitation spectrum of a ketocyanine dye has been studied at 77 K in pure solvents (methanol, ethanol, 2-methyltetrahydrofuran (2MTHF)) and in several mixed solvents. In a pure solvent at 77 K a single fluorescence band appears. In protic solvents the band gets reduced in intensity on melting and a red shifted emission band is obtained. On the other hand a blue shifted band appears on melting for aprotic solvents. This has been explained as due to a geometry change on excitation. Two emitting species have been identified in a glass containing mixed protic + aprotic solvents at 77 K. Preferential solvation of the solute by ethanol over 2MTHF has been indicated in a mixed binary solvent containing these two solvents.

Interchain photoluminescence in substituted polyfluorenes

We have characterised the interchain emitting states of two substituted polyfluorenes with steady-state and time-resolved photoluminescence (PL). Site selective excitation of the interchain state shows that physical dimers are formed in the film. These dimers have low concentration in the films but exciton diffusion to the dimers is reducing the intrachain exciton lifetime. With asymmetrical substituents, the PL quantum yield is higher and the PL lifetime is longer due to lower concentration of dimers. Slow cooling from the melt, gives a new red shifted emission band and less intrachain emission. A discrepancy in the calculated radiative lifetimes suggest a fast formation of this new band.

Effects of the Sol-Gel Processing on the Fluorescence Properties of Laser Dyes in Tetraethoxysilane Derived Matrices

Two fluorescent dyes were incorporated into sol-gel derived SiO2 matrices. The dye was added to SiO2 precursors of different degrees of pre-condensation and the spectroscopic properties of the immobilized dye were measured at various aging and drying stages of the resulting gels. The significant influence of the processing parameters on the spectroscopic properties is manifested in the relative intensity of a second red shifted emission band (550–560 nm), which was observed besides the typical coumarin emission band (495 nm). The appearance of this long wavelength emission might be attributed to dye aggregation or to other reactions with the ambient matrix forced by micro porosity phenomena.

Supersonic Jet Studies on the Photophysics of 4-Dimethylaminobenzonitrile (DMABN) and Related Compounds: on the Origin of the Anomalous Fluorescence of DMABN Clusters in a Supersonic Jet

Dispersed fluorescence and resonant one-color, two-photon ionization spectra of clusters of six derivatives of 4-dimethylaminobenzonitrile (DMABN) expanded in a supersonic jet are reported. The studied clusters comprise molecules, that show dual fluorescence in solution and others, that do not. The experimental setup allows one to obtain information on the cluster size distribution in the pulsed supersonic expansion by time-of-flight mass spectrometry and to correlate the mass spectra with the species contributing to the dispersed fluorescence emission spectrum. Under jet-cooled conditions, no red-shifted fluorescence is observed for the bare chromophore of all compounds under study except 4-(dimethylamino)-3,5-dimethylbenzonitrile (TMCA). The formation of homogeneous clusters (dimers or small clusters) gives rise to an additional red-shifted emission band, which is attributed to the formation of excimers. The stabilization of the excimer arises from both exciton resonance and charge resonance interaction. The phenomenon of dual fluorescence of aminobenzonitriles in dilute polar solutions is usually attributed to an intramolecular charge transfer induced by a twist of the donor group around the bond between the donor and acceptor moiety [twisted intramolecular charge transfer (TICT)]. Because the red-shifted fluorescence is observed even for the molecules, that exhibit no dual fluorescence in solution, this mechanism cannot account for the observations made in this study. Fluorescence detection with a charge coupled device (CCD) camera in combination with an image intensifier shows the enrichment of the heavier clusters on the jet axis in a supersonic expansion. This angular dispersion of the species with different mass affords special techniques for correlating mass and fluorescence spectra.

Photophysics of triphenylphosphines and their oxides: role of dimethylamino substituents

Steady-state and time-resolved spectroscopy experiments with picosecond or subpicosecond resolution are carried out in order to clarify the anomalous fluorescence properties of solutions prepared with triphenylphosphine compounds in various non-deaerated solvents at room temperature. Evidence is given for the formation of triphenylphosphine oxides in the solutions and for the coexistence of the oxidized and non-oxidized forms at the concentrations used for time-resolved experiments. The photoinduced formation of an emissive charge-transfer state, previously reported for solutions prepared with the dimethylamino substituted triphenylphosphines, is shown to be due to the corresponding oxide which exhibits a dual fluorescence with large solvatochromism. The non-oxidized forms of both the dimethylamino substituted and unsubstituted triphenylphosphines are found to exhibit a strongly red-shifted emission band with little solvatochromism. A photoinduced geometrical change with an increase of the conjugation in the excited state is proposed to explain this observation. On the other hand, an extra fluorescence band occuring at shorter wavelengths for the non-oxidized dimethylamino-substituted compound is not completely understood.