Structural and electronic properties of phosphino(oligothiophene) gold(I) complexes

A series of gold(I) complexes containing phosphino(oligothiophene) ligands of varying conjugation length has been prepared. Solid state crystal structures of (PT3)AuCl (PT3 = 5-diphenylphosphino-2,2':5',2' '-terthiophene) and AuCl(PTP)AuCl (PTP = 2,5-diphenylphosphinothiophene) have been obtained. The complex AuCl(PTP)AuCl crystallizes as a dimer with two intermolecular Au-Au contacts. Variable temperature NMR spectroscopy is used to demonstrate the presence of aurophilic interactions in solution for AuI(PTP)AuI. Dual emission is observed for AuCl(PTP)AuCl in solution and is attributed to emission from both monomer and dimer. In the solid state, dimer emission is dominant. The iodo analogue, AuI(PTP)AuI, shows only low energy dimer emission in both solution and the solid state. Compounds in which the ligands contain longer bridges (either bithienyl or terthienyl) show absorption and emission bands due to the pi-pi* transition only, both in solution and the solid state.     

Photophysical and electrochemical properties of Ru(II) complexes containing tridentate bisphosphino-oligothiophene ligands

Nine Ru(II) complexes containing the conjugated oligothiophene ligands 3,3'-bis(diphenylphosphino)-2,2':5',2'-terthiophene (P(2)T(3)) and 4',3'-bis(diphenylphosphino)-3,3'-dihexyl- 2,2':5',2':5',2':5',2'-pentathiophene (P(2)T(5)) were prepared and characterized. P(2)T(3) and P(2)T(5) bond as tridentate ligands and three of the complexes (1, 2 and 5) form green five-coordinate Ru(II) complexes in solution. Cyclic voltammetry, variable temperature UV-vis spectroscopy and time-resolved transient absorption spectroscopy were used to characterize the electronic properties of the complexes. Increased conjugation in the complexes containing the P(2)T(5) ligand resulted in a lowering of the oxidation potential of the oligothiophene, but electropolymerization was not observed. The electronic spectra were dominated by π-π* transitions. All of the complexes were non-emissive both at room temperature and low temperature, indicating the excited state decays by other, non-radiative pathways. The transient absorption spectrum of complex 7 shows a species with a band at 475 nm and a lifetime of ～100 ns, assigned to a ligand-based triplet state.     

Femtosecond study on the isomerization dynamics of NK88. II. Excited-state dynamics

The molecule 3,3(')-diethyl-2,2(')-thiacyanine isomerizes after irradiation with light of the proper wavelength. After excitation, it undergoes a transition, in which one or more conical intersections are involved, back to the ground state to form different product photoisomers. The dynamics before and directly after the transition back to the ground state is investigated by transient absorption spectroscopy in a wavelength region of 360-950 nm, as well as by fluorescence upconversion. It is shown that the excited-state dynamics are governed by two time scales: a short one with a decay time of less than 2 ps and a long one with about 9 ps. A thorough comparison of the experimental results with those of configuration interaction singles and time-dependent density functional theory calculations suggests that these dynamics are related to two competing pathways differing in the molecular twisting on the excited surface after photoexcitation. From the experimental point of view this picture arises taking into account the time scales for ground-state bleach, excited-state absorption, stimulated emission, fluorescence, and assumed hot ground-state absorption both in the solvent methanol and ethylene glycol.     

Dinitro and quinodimethane derivatives of terthiophene that can be both oxidized and reduced. Crystal structures,spectra, and a method for analyzing quinoid contributions to structure

Two new oligothiophenes, the dinitro compound 3',4'-dibutyl-5,5' '-dinitro-2,2':5',2' '-terthiophene (1) and the quinodimethane 3',4'-dibutyl-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene (2), have been synthesized and studied with electrochemistry, UV-vis-NIR-IR spectroscopy, ESR, and X-ray crystallography. These compounds, designed to be both electron and hole carriers, show redox properties that are unusual for oligothiophenes. Cyclic voltammetry and spectroelectrochemistry demonstrated that each compound could be oxidized to a cation radical and reduced to an anion radical and dianion. The spectra of 2 and its three redox partners were analyzed in terms of a limiting structure in which the neutral 2 has orbitals corresponding to those of a substituted-terthiophene dication. Compound 1 crystallizes with the thiophene rings held in an unusual nonplanar, cisoid configuration in face-to-face pi-stacks, with a spacing between molecules of 3.65 A. The C-C bond lengths of the outer nitro-substituted rings have quinoid character. Compound 2 crystallizes with the thiophene rings in a planar, transoid configuration. The molecules are held in pi-stacks formed from pi-dimers with a spacing between molecules of 3.47 and 3.63 A. The C-C bond distances of the thiophene rings of 1 and 2 and other oligomers were analyzed by a principal component analysis. The analysis found that 93% of the structural variance resided in one principal component related to the quinoid structure of the oligothiophene moiety. The analysis reliably demonstrated a quinoid contribution to the structure of 1. This method should be applicable to understanding the structure of other conjugated molecules in which quinoid structures contribute.     

Raman spectroscopy of short-lived terthiophene radical cations generated by photochemical and chemical oxidation.

The Raman spectra of various terthiophene radical cations are investigated; namely those of unsubstituted terthiophene and two styryl-substituted terthiophenes. Transient pump-probe resonance Raman spectroscopy is used to measure the short-lived radical cation spectra of non-end-capped 2,2':5',2'-terthiophene (3T) and 3'-[(E)-2-(4-nitrophenyl)ethenyl]-2,2':5',2'-terthiophene (NO2-pe3T). For these two compounds, the radical cations are generated via either direct photogeneration or photochemically using the electron acceptor tetracyanoethylene. The radical cation of 5,5'-dimethyl-3'-[(E)-2-phenylethenyl]-2,2':5',2'-terthiophene (DM-pe3T) is stable for up to five minutes as a result of the two alpha end caps and continuous-wave resonance Raman spectroscopy and chemical oxidation is used to obtain the spectrum of this radical cation. The resonance Raman spectra of all three terthiophene radical cations are dominated by a group of very intense bands in the low-frequency region. These bands have been assigned, by density functional theory methods, to C-S stretching modes coupled to thiophene ring deformations. These modes are significantly less intense in the sigma-dimer of NO2-pe3T [i.e. the corresponding styryl sexithiophene (NO2-pe3T)2]. This observation is attributed to a smaller change in the C--S bond order in the sexithiophene compared to the analogous terthiophene. This bond order difference may be rationalised by consideration of the singly occupied molecular orbital and lowest unoccupied molecular orbital, which are involved in the electronic transition probed by the laser excitation wavelength.     

Effect of molecular mass on supramolecular organisation of poly(4,4'-dioctyl-2,2':5',2'-terthiophene)

The effect of the chain length on the type and extent of the 2D supramolecular organization in poly(4,4'-dioctyl-2,2':5',2'-terthiophene) (PDOTT) monomolecular layers deposited on highly oriented pyrolytic graphite (HOPG) is studied by scanning tunneling microscopy (STM) and analyzed in terms of molecular modeling. The strictly monodispersed fractions of increasing molecular mass used in this study were obtained by chromatographic fractionation of the crude product of 4,4'-dioctyl-2,2':5',2'-terthiophene oxidative polymerization. STM investigations of PDOTT layers, deposited on HOPG from poly- and monodispersed fractions, show that polydispersity can be considered as a key factor seriously limiting supramolecular ordering. This is a consequence of significant differences in the type of supramolecular order observed for molecules of different chain length. It has been demonstrated that shorter molecules (consisting of 6 and 9 thiophene units) form well-defined two-dimensional islands, while the interactions between longer molecules (consisting of 12 and 15 thiophene units) become anisotropic. Consequently, for higher molecular mass fractions, the supramolecular organization is one-dimensional and consists of more or less separated rows of ordered macromolecules. In this case an increase of the chain length leads to amplification of the intermolecular interactions proceeding via interdigitation of the alkyl substituents of adjacent molecules. Polydispersed fractions show much less ordered organization because of the incompatibility of the supramolecular structures of molecules of different molecular masses. This finding is of crucial importance for the application of polythiophene derivatives in organic and molecular electronics since ordered supramolecular organization constitutes the condition sine qua non of good electrical transport properties.     

Reversible molecular switching of ruthenium bis(bipyridyl) groups bonded to oligothiophenes: effect on electrochemical and spectroscopic properties

We report the preparation of complexes in which ruthenium(II) bis(bipyridyl) groups are coordinated to oligothiophenes via a diphenylphosphine linker and a thienyl sulfur (P,S bonding) to give [Ru(bpy)(2)PT(3)-P,S](PF(6))(2) (bpy = 2,2'-bipyridyl, PT(3) = 3'-(diphenylphosphino)-2,2':5',2' '-terthiophene), [Ru(bpy)(2)PMeT(3)-P,S](PF(6))(2) (PMeT(3) = 3'-(diphenylphosphino)-5-methyl-2,2':5',2' '-terthiophene), [Ru(bpy)(2)PMe(2)T(3)-P,S](PF(6))(2) (PMe(2)T(3) = 5,5' '-dimethyl-3'-(diphenylphosphino)-2,2':5',2' '-terthiophene), and [Ru(bpy)(2)PDo(2)T(5)-P,S](PF(6))(2) (PDo(2)T(5) = 3,3' ' '-didodecyl-3' '-diphenylphosphino-2,2':5',2' ':5' ',2' ':5' ',2' ' '-pentathiophene). These complexes react with base, resulting in the complexes [Ru(bpy)(2)PT(3)-P,C]PF(6), [Ru(bpy)(2)PMeT(3)-P,C]PF(6), [Ru(bpy)(2)PMe(2)T(3)-P,C]PF(6), and [Ru(bpy)(2)PDo(2)T(5)-P,C]PF(6), where the thienyl carbon is bonded to ruthenium (P,C bonding). The P,C complexes revert back to the P,S bonding mode by reaction with acid; therefore, metal-thienyl bonding is reversibly switchable. The effect of interaction of the metal groups in the different bonding modes with the thienyl backbone is reflected by changes in alignment of the thienyl rings in the solid-state structures of the complexes, the redox potentials, and the pi --> pi transitions in solution. Methyl substituents attached to the terthiophene groups allow observation of the effect of these substituents on the conformational and electronic properties and aid in assignments of the electrochemical data. The PT(n)() ligands bound in P,S and P,C bonding modes also alter the electrochemical and spectroscopic properties of the ruthenium bis(bipyridyl) group. Both bonding modes result in quenching of the oligothiophene luminescence. Weak, short-lived Ru --> bipyridyl MLCT-based luminescence is observed for [Ru(bpy)(2)PDo(2)T(5)-P,S](PF(6))(2), [Ru(bpy)(2)PT(3)-P,C]PF(6), [Ru(bpy)(2)PMeT(3)-P,C]PF(6), and [Ru(bpy)(2)PMe(2)T(3)-P,C]PF(6), and no emission is observed for the alternate bonding mode of each complex.     

Syntheses, and optical, fluorescence, and nonlinear optical characterization of phosphine-substituted terthiophenes

Earlier studies of phosphine-substituted terthiophenes have demonstrated that some of these materials exhibit nonlinear absorption at 532 nm. However, this wavelength is significantly removed from the linear absorption maxima of the complexes, suggesting that better nonlinear absorption might be observed at wavelengths closer to the linear absorption maxima. To investigate this possibility, a library of compounds has been prepared either by varying the group attached to the nonbonding pair of electrons on the phosphorus atoms of 5,5'-bis(diphenylphosphino)-2,2':5',2'-terthiophene (PT(3)P), or by introducing additional substituents on the 5'-position of 5-(diphenylphosphino)-2,2':5',2'-terthiophene (PT(3)). All these compounds have been characterized using multinuclear NMR, UV-vis, and fluorescence spectroscopy. The compounds are strongly fluorescent, and both the fluorescence wavelength and the intensity depend upon the thiophene substituents. The nonlinear optical properties have also been evaluated at various wavelengths in the blue region. Each compound exhibits reverse saturable absorption, and the intensity of the reverse saturable absorption at a particular wavelength depends on the chemical structure of the compound.     

Pyrene excimer signaling molecular beacons for probing nucleic acids

Molecular beacon DNA probes, containing 1-4 pyrene monomers on the 5' end and the quencher DABCYL on the 3' end, were engineered and employed for real-time probing of DNA sequences. In the absence of a target sequence, the multiple-pyrene labeled molecular beacons (MBs) assumed a stem-closed conformation resulting in quenching of the pyrene excimer fluorescence. In the presence of target, the beacons switched to a stem-open conformation, which separated the pyrene label from the quencher molecule and generated an excimer emission signal proportional to the target concentration. Steady-state fluorescence assays resulted in a subnanomolar limit of detection in buffer, whereas time-resolved signaling enabled low-nanomolar target detection in cell-growth media. It was found that the excimer emission intensity could be scaled by increasing the number of pyrene monomers conjugated to the 5' terminal. Each additional pyrene monomer resulted in substantial increases in the excimer emission intensities, quantum yields, and excited-state lifetimes of the hybridized MBs. The long fluorescence lifetime ( approximately 40 ns), large Stokes shift (130 nm), and tunable intensity of the excimer make this multiple-pyrene moiety a useful alternative to traditional fluorophore labeling in nucleic acid probes.     

Exploration of ground and excited electronic states of aromatic and quinoid S,S-dioxide terthiophenes. Complementary systems for enhanced electronic organic materials

We analyze the electronic and molecular structures for the ground and excited electronic states of aromatic terthiophene (3T), the quinodimethane 3',4'-dibutyl-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene (3Q), and isologues with the middle ring S-oxidized (3TO2, 3QO2). These represent extremes of electron rich and deficient ground states, often exhibiting complementary properties. Oxidizing the central sulfur atom affects the molecular structure, electron affinity, and photophysical properties of both pi systems. The consequences for 3T include de-aromatization of the central thiophene, red-shifting of the electronic absorption spectrum, and lowering of the reduction potential. The electron deficient quinoid 3QO2 shows an enhancement of electron affinity from reducing the electron-donor ability of sulfur, and a blue-shifting of its electronic absorption spectrum was seen. Fluorescence emission is quenched in the sulfonated terthiophene, and the contrary effect again would be expected upon sulfonation of a quinoid emitter. Raman vibrational spectroscopy, electrochemistry, and UV-vis and fluorescence spectroscopies are analyzed in conjunction with theoretical calculations.     

Structure and dynamics of the triplet state of oligothiophenes in isotropic and partially oriented matrices

The photoexcited triplet states of 4,4'-dipentoxy-2,2'-dithiophene (4-T2), 3,3'-dipentoxy-2,2'-dithiophene (3-T2), and 4,4'-dipentoxy-2,2':5',2':5',2'-tetrathiophene (4-T4) have been investigated by time-resolved electron paramagnetic resonance in glassy toluene and in a frozen oriented liquid crystal, which provides a partially ordered medium. The preferential orientation of the rod-like 4-T2 and 4-T4 is compared to that of the disk-like 3-T2. The use of an oriented matrix coupled to simple semiempirical calculations allowed us to determine the orientation of the principal axes of the fine interaction with respect to the molecular axes. The motional behavior of the molecules in the isotropic and anisotropic matrices has been studied by comparing the spectral profiles of the Echo detected EPR (Echo-EPR) spectra with those of the continuous wave time resolved EPR (TR-EPR). A model considering the modulation of the zero-field splitting (ZFS) by molecular libration accounts for the Echo-EPR lineshape, on the basis of motions around preferential axes depending on the embedding matrix. The differences in the ZFS parameters of the two isomers 4-T2 and 3-T2 are attributed to a mesomeric effect due to the substituents.     

Synthesis, characterization, and fluorescence behavior of twisted and planar B2N2-quaterphenyl analogues

A series of planar and twisted heteroaromatic quaterphenyl analogues containing BN ring linkages has been synthesized using primarily difunctional Lewis acidic diborabiphenyl moieties as molecular cores. Crystal structure analyses indicated the presence of large twist angles between adjacent aromatic rings in 1 and 3, which were also observed to possess nonfluorescent behavior due to a lack of molecular rigidity and insufficient B=N character in the excited state. In contrast, the incorporation of one or two bridging ethylene groups between the adjacent rings (installed via an ethynyl cycloisomerization) was found to afford planar phenanthrene or pyrene moieties, which resulted in weak fluorescence behavior (Phi F = 0.02-0.16) for the n-Bu and Ph derivatives 5-12. Emission colors ranged from green (lambda em = 521 nm) to red (lambda em = 630 nm) and depended primarily on the conformation (2,2'- vs 4,4'-), the extent of chromophore conjugation (phenanthrene vs pyrene), and the type of exocyclic substituent present (n-Bu vs Ph). Communication between the two phenanthrene or pyrene moieties was observed in some cases, which was characterized by bathochromically shifted emission bands relative to that of monomeric phenanthrene or pyrene species. Unique excited-state dimer (excimer) fluorescence was observed for the 2,2'-isomer 8, which was characterized by broad, low-energy emission bands bathochromically shifted from that of the corresponding monomer.     

Electropolymerization of Pd(II) complexes containing phosphinoterthiophene ligands

A series of Pd complexes of 3'-diphenylphosphino-2,2':5'2' '-terthiophene (1a, dppterth) in which the metal is coordinated in three different modes have been prepared and electropolymerized, resulting in the formation of conductive thin films. In [Pd2(mu-Cl2)(dppterth-P,C3)2] (3a) the metal is P,C-coordinated, in [PdCl2(dppterth-P)2] (4a) the coordination is monodentate via the phosphine, and in [Pd(dppterth-P,C3)(dppterth-P,S1)][PF6] (5a) both P,C- and P,S-coordination modes are found. In 5a, the coordinated thiophene is hemilabile and may be displaced by reaction with more strongly coordinating ligands such as isocyanides. To probe the effect of blocking the alpha-position of the terthienyl moiety with methyl groups, 3'-diphenylphosphino-5-methyl-2,2':5'2' '-terthiophene (1b, Me-dppterth) and 3'-diphenylphosphino-5,5' '-dimethyl-2,2':5'2' '-terthiophene (1c, Me2-dppterth) were prepared, and the corresponding series of Pd complexes was synthesized. One of these complexes, [Pd(Me2-dppterth-P,C3)(Me2-dppterth-P,S1)][PF6] (5c), has been crystallographically characterized. The electropolymerized films prepared from 5a react with isonitriles, and shifts in the absorption spectra of the electropolymerized materials are observed upon reaction. A Pd complex has also been prepared from 5-diphenylphosphino-2,2':5'2' '-terthiophene (2, 5dppterth), and this complex has been electropolymerized. All the electropolymerized thin films have been characterized using EDX analysis, which demonstrates good correspondence with the elemental analysis of the respective monomers, and the maximum conductivities of the films are near 10(-4) S x cm(-1). Comparing the electropolymerization behavior of the complexes, along with their electrochemical and spectroscopic data, allows conclusions to be drawn regarding the involvement of pi-delocalization and the metal group in the conductivity of the materials.     

Molecular weight dependent charge carrier mobility in poly(3,3' '-dioctyl-2,2':5',2' '-terthiophene)

Poly(3,3' '-dioctyl-2,2':5',2' '-terthiophene), a polymer recently used for the fabrication of organic field effect transistors, has been fractionated into five fractions distinctly differing in their molecular weights (Mn), with the goal of determining the influence of the degree of polymerization (DPn) on its principal physicochemical parameters. It has been demonstrated that within the Mn range studied (from 1.5 kDa to 10.5 kDa by SEC), corresponding to DPn from 10 to 38, the polymer band gap steadily decreases with growing molecular weight, which is clearly manifested by an increasing bathochromic shift of the band originating from the pi-pi* transition. The same trend is observed for the HOMO level, determined from the onset of the p-doping in cyclic voltammetry, which shifts from -5.10 eV to -4.90 eV for the lowest and the highest molecular weight fractions, respectively. The most pronounced influence of DPn has been found for the charge carriers' mobility-one of the most important parameters of field effect transistors (FETs) fabricated from this polymer. A fourfold increase in DPn results in an increase of the carriers' mobility by more than 3 orders of magnitude. Comparison of these results with those obtained for fractionated regioregular poly(3-hexylthiophene) shows a strikingly similar behavior of both polymers with respect to the molecular weight.     

Photo-induced transformations in 2,2':5',2'-terthiophene thin films on silver

We report on transient photo-induced structural changes of 2,2':5',2'-terthiophene (3T) adsorbed on silver nanoparticles. Surface-enhanced Raman scattering (SERS) data show that the film thickness predetermines the eventual course of these changes. In particular, for high molecular concentrations (thick adsorbate layers), illumination leads to a photoinduced polymerization of the 3T film. For thin layers ( approximately monolayer coverage), the photoinduced spectral evolution is qualitatively different. We interpret the latter process as a cleavage of the terthiophene molecule, resulting in isolated thiophene units. Calculations of excitation energies for 3T + 3Ag+ and 1T + Ag+ revealed visible transitions, while calculated Raman frequencies indicated 1T as a possible photoproduct. As no photoinduced transformations are observed for 3T in the solid or solution phases, it is likely that the processes reported here originate in surface-enhanced charge-transfer between the Ag-surface and the organic adsorbate.     

Identifiability of models for intramolecular two-state excited-state processes with added quencher and coupled species-dependent rotational diffusion

The parameters describing the kinetics of excited-state processes can possibly be recovered by analysis of the fluorescence decay surface measured as a function of the experimental variables. The identifiability analysis of a photophysical model assuming errorless time-resolved fluorescence data can verify whether the model parameters can be determined and may lead to the minimal experimental conditions under which this is possible. In this work, we used the method of similarity transformation to investigate the identifiability of three kinetic models utilized to describe the time-resolved fluorescence of reversible intramolecular two-state excited-state processes in isotropic environments: (1) model without added quencher, (2) model with added quencher, (3) model with added quencher coupled with species-dependent rotational diffusion described by Brownian reorientation. Without a priori information, model 1 is not identifiable. For model 2, two sets of quenching rate constants and combinations of excited-state deactivation/exchange rate constants are possible, but they cannot be allocated to a specific excited-state species. For both sets, upper and lower limits on the excited-state deactivation/exchange rate constants can be obtained. For model 3, both spherically and cylindrically symmetric rotors, with no change in the principal axes of rotation in the latter, are considered. The fluorescence delta-response functions I(parallel)(t) and I(perpendicular)(t), for fluorescence polarized parallel and perpendicular, respectively, to the electric vector of linearly polarized excitation, are used to define the sum S(t) identically equal to I(parallel)(t) + 2 I(perpendicular)(t) and the difference D(t) identically equal to I(parallel)(t) - I(perpendicular)(t). The identifiability analysis is performed using the S(t) and D(t) functions. Also for model 3, two sets of kinetic parameters (i.e., quenching rate constants, combinations of deactivation/exchange rate constants, and rotational diffusion coefficients) exist, but these parameters cannot be assigned unequivocally to a specific species. For the three models, an infinite number of alternative spectroscopic parameters associated with excitation and emission are found.     

Creation of Functional Organic Gels Based on Oligothiophenes

Polymer gels have received a great deal of attention not only from scientific interest but also for their practical applications. Recently, low molecular-weight organic gels have also been receiving growing attention. However, their have been few studies of low molecular-weight organic gels in contrast to extensive studies of polymer gels. In order to develop a novel class of low-molecular-weight organic gels and to gain an insight into the relationship between molecular structures of gel-form…     

Push-pull effects and emission from ternary complexes of platinum(II), substituted terpyridines, and the strong-field cyanide ion

As part of an effort to develop new lumaphors involving late transition metal ions, this report describes the synthesis and characterization of the first platinum(II) derivatives containing 2,2':6',2'-terpyridine (trpy) and cyanide as co-ligands. According to existing models, including cyanide in the coordination sphere should raise the energies and minimize the influence of short-lived d-d excited states that otherwise compromise the excited-state lifetime. Both [Pt(trpy)(CN)]+ and the 4'-cyano-2,2':6',2'-terpyridine analogue [Pt(CN-T)(CN)]+ are emissive in dichloromethane solution, but the signals are weak. Part of the problem is that the d-pi* charge-transfer excited states also rise in energy, so that the emission actually originates from a (3)pi-pi* state with a relatively low radiative rate constant. However, another member of the series, the 4'-dimethylamino-2,2':6',2'-terpyridine (dma-T) derivative [Pt(dma-T)(CN)]+, proves to be a very promising platform with an emission quantum yield of phi= 0.26 and an excited-state lifetime of tau = 22 micros in room-temperature, deoxygenated dichloromethane solution. In the dma-T complex the electron-rich dimethylamino substituent provides the basis for an emissive, but largely ligand-based, charge-transfer excited state. The orbital parentage is such that the photoluminescence persists in donating solvents like dimethylformamide, which ordinarily quenches d-pi* excited states in complexes of this type.     

Geometric and excited-state properties of 1,4-bis(benzothiazolylvinyl)benzene interacting with 2,2',2' '-(1,3,5-phenylene)tris[1-phenyl-1H-benzimidazole] studied by a density-functional tight-binding method

The energetics and luminescent property of a guest molecule, 1,4-bis(benzothiazolylvinyl)benzene (BT), interacting with a host molecule, 2,2',2' '-(1,3,5-phenylene)tris[1-phenyl-1H-benzimidazole] (TPBI), in organic light-emitting diodes are studied by performing excited-state calculations using a time-dependent density-functional tight-binding method complemented with dispersion energy. It is found that the overlap between the TPBI emission and the BT absorption spectra shows an efficient energy transfer from the host molecule to the guest molecule when they are excited. The planar BT molecule becomes distorted when it is mixed with TPBI, resulting in a blue luminescence around 475 nm. The separation of the TPBI + BT mixture on a graphite surface is found to be energetically favorable, consistent with experimental observation.     

Excited-State Raman Spectroscopy of Inorganic Compounds

Abstract— Raman spectra of inorganic complexes in excited electronic states are discussed. A brief overview of the field of transient Raman spectroscopy and experimental considerations are presented. Two examples from the author's laboratory are used to illustrate the type of information that can be obtained. The first example, an excited-state Raman spectroscopic study of K₃[Mn(CN)₅NO], is chosen because it illustrates the connections between excited-state molecular structure and vibrational properties. The pump pulse causes a change from a linear sp-hybridized NO containing a triple bond to a bent sp²-hybridized NO containing a double bond. Both the NO stretch and normal modes involving other ligands are measured and interpreted. The second example is chosen to illustrate the vibrational consequences of photoinduced electron transfer. The Raman spectra of W(CO)₄(diimine) complexes (diimine = 2,2'-bipyridine, 4,4'-dimethyl-2,2'-bipyridine, and isopropyl-pyridine-2-carbaldehyde imine) in the lowest tungsten to diimine charge transfer excited state are discussed. The excited-state peaks are assigned to ligand ring deformation modes and to carbonyl stretching modes. The totally symmetric cis -carbonyl stretching mode in the charge transfer excited state is about 50 cm' higher in energy than that of the molecule in the ground electronic state. The increase is interpreted in terms of loss of metal-car-bonyl back-bonding in the charge transfer excited state. Finally, a summary of the field's strengths and difficulties and a brief discussion of the future perspectives are presented.