Acid-base sensors based on novel quinone-type dyes

We present a detailed study on the acid-base behaviour of a family of "potentially antiaromatic" p-benzoquinonediimine ligands. These 12pi electron molecules can be considered as constituted of two chemically connected but electronically not conjugated 6pi-electron subunits. Upon successive protonation, "mono" and "double" cyanine-type chromophores are generated in solution and allow a precise and sensitive spectrophotometric detection. These molecules represent a new class of tunable quinones whose electronic and structural properties can be triggered by proton input, as established by a complete physico-chemical study involving a combination of potentiometric and spectrophotometric methods (absorption and emission).     

Fluorine as a pi donor. Carbon 1s photoelectron spectroscopy and proton affinities of fluorobenzenes

The carbon 1s ionization energies for all of the carbon atoms in 10 fluorine-substituted benzene molecules have been measured by high-resolution photoelectron spectroscopy. A total of 30 ionization energies can be accurately described by an additivity model with four parameters that describe the effect of a fluorine that is ipso, ortho, meta, or para to the site of ionization. A similar additivity relationship describes the enthalpies of protonation. The additivity parameters reflect the role of fluorine as an electron-withdrawing group and as a pi-electron donating group. The ionization energies and proton affinities correlate linearly, but there are four different correlations depending on whether there are 0, 1, 2, or 3 fluorines ortho or para to the site of ionization or protonation. That there are four correlation lines can be understood in terms of the ability of the hydrogens at the site of protonation to act as a pi-electron acceptor. A comparison of the ionization energies and proton affinities, together with the results of electronic structure calculations, gives insight into the effects of fluorine as an electron-withdrawing group and as a pi donor, both in the neutral molecule and in response to an added positive charge.     

An ab initio valence bond study on cyclopenta-fused naphthalenes and fluoranthenes

To probe the effect of external cyclopenta-fusion on a naphthalene core, ab initio valence bond (VB) calculations have been performed, using strictly atomic benzene p-orbitals and p-orbitals that are allowed to delocalize, on naphthalene (1), acenaphthylene (2), pyracylene (3), cyclopenta[b,c]acenaphthylene (4), fluoranthene (5), and cyclopenta[c,d]fluoranthene (6). For the related compounds 1-4 and 5,6 the total resonance energies (according to Pauling's definition) are similar. Partitioning of the total resonance energy in contributions from the possible 4n + 2 and 4n pi-electron conjugated circuits shows that only the 6pi-electron conjugated circuits (benzene-like) contribute to the resonance energy. The results show that cyclopenta-fusion does not extend the pi system in the ground state; the five-membered rings act as peri-substituents. As a consequence, the differences in (total) resonance energy do not coincide with the differences in thermodynamic stability. Notwithstanding, the relative energies of the Kekule structures can be estimated using Randic's conjugated circuits model.     

DNA‐Based Oligochromophores as Light‐Harvesting Systems

The chromophores ethynyl pyrene as blue, ethynyl perylene as green and ethynyl Nile red as red emitter were conjugated to the 5‐position of 2′‐deoxyuridine via an acetylene bridge. Using phosphoramidite chemistry on solid phase labelled DNA duplexes were prepared that bear single chromophore modifications, and binary and ternary combinations of these chromophore modifications. The steady‐state and time‐resolved fluorescence spectra of all three chromophores were studied in these modified DNA duplexes. An energy‐transfer cascade occurs from ethynyl pyrene over ethynyl perylene to ethynyl Nile red and subsequently an electron‐transfer cascade in the opposite direction (from ethynyl Nile red to ethynyl perylene or ethynyl pyrene, but not from ethynyl perylene to ethynyl pyrene). The electron‐transfer processes finally provide charge separation. The efficiencies by these energy and electron‐transfer processes can be tuned by the distances between the chromophores and the sequences. Most importantly, excitation at any wavelength between 350 and 700 nm finally leads to charge separated states which make these DNA samples promising candidates for light‐harvesting systems.     

Property tuning in charge-transfer chromophores by systematic modulation of the spacer between donor and acceptor

A series of donor-acceptor chromophores was prepared in which the spacer separating 4-dimethylanilino (DMA) donor and C(CN)(2) acceptor moieties is systematically varied. All of the new push-pull systems, except 4 b, are thermally stable molecules. In series a, the DMA rings are directly attached to the central spacer, whereas in series b additional acetylene moieties are inserted. X-ray crystal structures were obtained for seven of the new, intensely colored target compounds. In series a, the DMA rings are sterically forced out of the mean plane of the residual pi system, whereas the entire conjugated pi system in series b is nearly planar. Support for strong donor-acceptor interactions was obtained through evaluation of the quinoid character of the DMA ring and by NMR and IR spectroscopy. The UV/Vis spectra feature bathochromically shifted, intense charge-transfer bands, with the lowest energy transitions and the smallest optical gap being measured for the two-dimensionally extended chromophores 6 a and 6 b. The redox behavior of the push-pull molecules was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). In the series 1 b, 2 b, 4 b, 5 b, in which the spacer between donor and acceptor moieties is systematically enlarged, the electrochemical gap decreases steadily from 1.94 V (1 b) to 1.53 V (5 b). This decrease is shown to be a consequence of a reduction in the D-A conjugation with increasing spacer length. Degenerate four-wave mixing experiments reveal high third-order optical nonlinearities, pointing to potentially interesting applications of some of the new chromophores in optoelectronic devices.     

Extreme projection of a proton into the pi-cloud of an aromatic ring: record shielding of an aromatic proton in trans-10b-methyl-10c-(1-naphthyl)-10b,10c-dihydropyrene

A synthetic sequence involving dithiametacyclophane --> metacyclophanediene --> dihydropyrene was employed to prepare trans-10b-methyl-10c-(2-naphthyl)- and trans-10b-methyl-10c-(1-naphthyl)-10b,10c-dihydropyrene 5 and 6, respectively. Both exhibit a strong diamagnetic ring current despite the introduction of an internal bulky substituent within the pi-electron cloud. Their electronic spectra suggest interaction between the two near-perpendicular naphthyl and dihydropyrenyl pi systems, resulting in red shift and band broadening. All naphthyl protons are well resolved in their 1H NMR spectra due to a strong shielding effect of the dihydropyrene ring. The most shielded protons in 5 and 6 are H1' and H2' at delta 2.47 and 1.42, respectively, being 5.25 and 5.95 ppm shifted from those of reference protons. There is evidence for free rotation on the NMR time scale of the 2-naphthyl ring in 5 with a preference for a particular conformer, whereas the 1-naphthyl ring in 6 is conformationally rigid with its H2' projecting deeply into the pi-cloud, thus accounting for the most shielded aromatic proton (H2' in 6) reported to date.     

Near‐Infrared Responsive Conjugated Polymers to 1.5 μm and Beyond: Synthesis and Electrochromic Switching Application

Azulene‐containing conjugated polymers with near‐infrared absorption up to 1.5 μm and beyond are achieved by treating with trifluoroacetic acid (TFA). Density functional theory calculations reveal that the near‐infrared absorption arises from a strong intramolecular charge transfer transition on the polymer backbone, and the near‐infrared absorption can be tuned by the degree of protonation. Furthermore, TFA treated polymers show a ten‐fold enhancement in electrochromic contrast and significantly improved switching stability, suggesting that these polymers are promising candidates for fabrication of the first generation organic near‐infrared devices.     

Attachment of styrene and phenylacetylene on Si(111)-7 x 7: the influence of substitution groups on the reaction mechanism and formation of pi-conjugated skeletons

The interactions of styrene and phenylacetylene and their isotope substitutions with a Si(111)-7 x 7 surface have been studied as model systems to mechanistically understand the chemical binding of conjugated pi-electron systems to di-radical-like silicon dangling bonds of the adjacent adatom-rest atom pair. Vibrational studies show that styrene mainly binds to the surface through a diradical reaction involving both the external C=C and its conjugated internal C=C of the phenyl ring with an adjacent adatom-rest atom pair, forming a 5-ethylidene-1,3-cyclohexadiene-like skeleton. On the other hand, phenylacetylene was shown to be covalently attached to Si(111)-7 x 7 through the external C[triple bond]C, forming a styrene-like conjugation system. These experimental results are consistent with density functional theory calculations. The different binding mechanisms for styrene and phenylacetylene clearly demonstrate that reaction channels for multifunctional organic molecules are strongly dependent on the chemical and physical properties of the functional groups. The resulting pi-electron conjugation structures may possibly be employed as intermediates for further organic syntheses and fabrication of multilayer organic films on semiconductor surfaces.     

How chromophore shape determines the spectroscopy of phenylene-vinylenes: origin of spectral broadening in the absence of aggregation

Single oligo(phenylene-vinylene) molecules constitute model systems of chromophores in disordered conjugated polymers and can elucidate how the actual conformation of an individual chromophore, rather than that of an overall polymer chain, controls its photophysics. Single oligomers and polymer chains display the same range of spectral properties. Even heptamers support pi-electron conjugation across approximately 80 degrees curvature, as revealed by the polarization anisotropy in excitation and supported by quantum chemical calculations. As the chain becomes more deformed, the spectral linewidth at low temperatures, often interpreted as a sign of aggregation, increases up to 30-fold due to a reduction in photophysical stability of the molecule and an increase in random spectral fluctuations. The conclusions aid the interpretation of results from single-chain Stark spectroscopy in which large static dipoles were only observed in the case of narrow transition lines. These narrow transitions originate from extended chromophores in which the dipoles induced by backbone substituents do not cancel out. Chromophores in conjugated polymers are often thought of as individual linear transition dipoles, the sum of which make up the polymer's optical properties. Our results demonstrate that, at least for phenylene-vinylenes, it is the actual shape of the individual chromophore rather than the overall chromophoric arrangement and form of the polymer chain that dominates the spectroscopic properties.     

The mutual influence of the chromophores of two cyanine dyes connected through a benzene nucleus

Three isomeric biscyanine dyes have been synthesized, each of which forms a pair of indothiazolocarbocyanines attached to a p-phenylene group through the 4,4′ (I), 5, 5′ (II), and 3, 3′ (III) positions of the thiazole nuclei. It has been found that in the biscyanines II and III the polymethine chromophores are conjugated and influence one another, which is shown by the splitting of the absorption bands of these dyes. In the biscyanines I, no interaction of the chromophores is observed.     

Electronic structure and spectra of catechol and alizarin in the gas phase and attached to titanium

Ab initio electronic structure calculations elucidate the dramatic differences observed in the electronic spectra of the catechol and alizarin molecules upon binding to titanium. Catechol and alizarin are similar chromophores with analogous electronic spectra in the free state. Binding alizarin to titanium red-shifts the spectrum. The binding of catechol to titanium produces a new optically active transition, at the same time preserving the features of the free catechol spectrum. By examining the details of the calculations, we can rationalize the spectral differences in the catechol and alizarin systems by the positioning of the catechol and alizarin pi molecular orbitals relative to the conduction band of TiO(2).     

Effects of conjugation length and resonance enhancement on two-photon absorption in phenylene-vinylene oligomers

Two-photon excitation spectra have been recorded over the large spectral range of 540-1000 nm for five phenylene-vinylene oligomers that differ in the length of the conjugated pi system. The significant changes observed in the two-photon excitation spectra and absorption cross sections as a function of this systematic change in the chromophore are discussed in light of (1) the corresponding one-photon absorption spectra and (2) high-level density functional response theory calculations performed on analogues of these systems. The results obtained illustrate one way to exploit parameters that influence nonlinear optical properties in large organic molecules. Specifically, data are provided to indicate that when the frequency of the laser used in the two-photon experiment is nearly-resonant with an allowed one-photon transition, significant increases in the two-photon absorption cross section can be realized. This phenomenon of the so-called resonance enhancement allows for greater control in obtaining an optimal response when using existing two-photon chromophores, and provides a much-needed guide for the systematic development and efficient use of two-photon singlet oxygen sensitizers.     

Geometric approximation for molecular polarizabiities

Molecular polarizabilities calculated with the geometric approximation are shown to be independent of the type of perturbation theory used. Agreement with finite field calculations, including anisotropies, is obtained for small molecules. A modified procedure for including sigma and pi contributions in a differentiated way is developed within the CNDO scheme. Applications to planar conjugated systems are displayed.     

Synthesis, properties, and reactions of a series of stable dialkyl-substituted silicon-chalcogen doubly bonded compounds

The first dialkyl-substituted silicon-chalcogen doubly bonded compounds [R2Si=X; R2=1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl, X = S (4), Se (5), and Te (6)]were synthesized by the reactions of an isolable dialkylsilylene R2Si: (3) with phosphine sulfide, elemental selenium, and elemental tellurium, respectively. Systematic changes of characteristics of silicon-chalcogen double bonds are elucidated by X-ray analysis, UV-vis spectroscopy, and DFT calculations. In the solid state, the unsaturated silicon atom in 4-6 adopts planar geometry and the extent of the shortening of Si=X double bonds from the corresponding Si-X single bonds decreases in the order 4 > 5 > 6. In the absorption spectra of 4-6, pi -->pi* transition bands are observed distinctly in addition to n -->pi* transition bands. Both the n -->pi* and pi -->pi* transitions are red-shifted in the order 4 < 5 < 6, and the difference between the energies of the two transitions is kept almost constant among 4-6. The tendency is explained using the qualitative perturbation theory and is reproduced by the DFT calculations for model silanechalcogenones. Addition reactions of water, methanol, and isoprene to 4-6 are reported.     

Adsorption geometry and core excitation spectra of three phenylpropene isomers on Cu(111)

Theoretical C 1s near edge x-ray absorption fine structure (NEXAFS) spectra for the C(9)H(10) isomers trans-methylstyrene, alpha-methylstyrene, and allylbenzene in gas phase and adsorbed at Cu(111) surfaces have been obtained from density functional theory calculations where adsorbate geometries were determined by corresponding total energy optimizations. The three species show characteristic differences in widths and peak shapes of the lowest C 1s-->pi(*) transitions which are explained by different coupling of the pi-electron system of the C(6) ring with that of the side chain in the molecules as well as by the existence of nonequivalent carbon centers. The adsorbed molecules bind only weakly with the substrate which makes the use of theoretical NEXAFS spectra of the oriented free molecules meaningful for an interpretation of experimental angle-resolved NEXAFS spectra of the adsorbate systems obtained in this work. However, a detailed quantitative account of relative peak intensities requires theoretical angle-resolved NEXAFS spectra of the complete adsorbate systems which have been evaluated within the surface cluster approach. The comparison with experiment yields almost perfect agreement and confirms the reliability of the calculated equilibrium geometries of the adsorbates. This can help to explain observed differences in the catalytic epoxidation of the three molecules on Cu(111) based on purely geometric considerations.     

Aromaticity of four-membered-ring 6pi-electron systems: N2S2 and Li2C4H4

N(2)S(2) is a four-membered-ring system with 6pi electrons. While earlier proposals considered N(2)S(2) to be aromatic, recent electronic structure calculations claimed that N(2)S(2) is a singlet diradical. Our careful reexamination does not support this assertion. N(2)S(2) is closed shell and aromatic since it satisfies all three generally accepted criteria for aromaticity: energetic (stability), structural (planarity with equal bond lengths), and magnetic (negative nucleus-independent chemical shift due to the pi electrons). These characteristics as well as the electronic structure of N(2)S(2) are compared with those for an isoelectronic pi system, Li(2)C(4)H(4), motivated by theoretical and recent experimental investigations that confirmed its aromaticity. However, N(2)S(2) and Li(2)C(4)H(4) are both essentially 2pi-electron aromatic systems with a formal N-S (C-C) bond order of 1.25 even though they both have 6pi electrons. This is because four of the six pi electrons occupy the nonbonding pi HOMOs and only two electrons participate effectively in the aromatic stabilization. However, wave function analysis shows relatively large LUMO occupation numbers; this antibonding effect can be said to reduce the aromatic character by approximately 7% and 4% for N(2)S(2) and Li(2)C(4)H(4), respectively.     

Tailoring transition metal complexes for nonlinear optics applications. 2. A theoretical investigation of the second-order nonlinear optical properties of M(CO)(5)L complexes (M = Cr, W; L = Py, PyCHO, Pyz, PyzBF(3), BPE, BPEBF(3))

In this work, we report an ab initio investigation of second-order nonlinear optical (NLO) properties and absorption electronic spectra of push-pull transition metal chromophores of the formula [M(CO)(5)L] (M = Cr, W; L = pyridine (Py), 4-formyl-pyridine (PyCHO), pyrazine (Pyz), trans-1,2-bis(4-pyridyl)ethylene (BPE)). Pyz and BPE are considered either with one nitrogen atom free or interacting with the strong acceptor BF(3). All of the molecular properties have been calculated using two different and methodologically independent approaches: the time dependent and coupled perturbed density functional theories (TDDFT and CPDFT) and the sum-over-states (SOS) approach, where the excited states are obtained via the single configuration interaction (SCI) ab initio method. DFT results are in acceptable agreement with the experimental energy values of electronic transitions (with the exception of chromophores with the large pi-delocalization, like BPE); SCI calculations overestimate excitation energies and produce an inversion in the order of d(M) --> pi(L) and d(M) --> pi(CO) transitions. The SCI-SOS approach gives first-order hyperpolarizabilities, basically in agreement as trend and values with the experiments and seems to be a tool generally suitable for the evaluation of these properties also for transition metal complexes. On the other hand, the first-order hyperpolarizabilities computed using the CPDFT approach are consistently overestimated in comparison with the experimental results, especially in the case of a ligand with large pi-delocalization. We also show that the "two-level" approximation taking into account only the lowest energy charge transfer excitation (e.g., d(M) --> pi(L)) is not applicable to chromophores with the extended pi-delocalized ligand (BPE) coordinated to a transition metal, due to significant contributions originating from intraligand pi(L) --> pi(L) transitions. This study reports a detailed analysis and comparison of electronic NLO effects of transition metal complexes computed with DFT and ab initio SCI-SOS methodology.     

Application of natural bond orbital analysis to delocalization and aromaticity in C-substituted tetrazoles.

Energies of two tautomeric forms of 10 tetrazole derivatives substituted at C5 were established by DFT/B3LYP calculations carried out at the 6-311++G level. In each case the calculated energy of the 2H-tautomer was lower than that of the 1H. Furthermore, three geometric aromaticity indices of both forms were calculated, as were the values of nuclear independent nuclear shift and aromatic stabilization energy. The electronic properties were evaluated with the help of the natural bonding orbital theory. Following this a new pi-delocalization parameter, the root-mean square of pi-electron density localized on the atoms of the five-membered tetrazole ring, SDn, was introduced. It was concluded that the electronic delocalization can be described equally well by three different parameters: SDn, the extent of the transfer of electron density from the p(z) orbital of one nitrogen to the rest of the pi electron system, and population of two antibonding pi orbitals. Arguably, the information provided by the electronic parameters is similar to that contained in the geometric (structural) aromaticity indices except for tetrazole substituted by -BH(2).