Biomedical subjects
Publications and source records attributed to Egbert Zojer.
Extended bis(donor)-substituted squaraine chromophores exhibit very high two-photon cross-sections (as high as 33 000 GM) in the near-IR; these can be attributed to the combination of large transition dipoles with small detuning energies. The modulus of the third-order nonlinear optical susceptibility at 1.3 mum has been found to be 7.0 x 10-11 esu for one of these chromophores.
Based on essential-state models for three-photon absorption (3PA), we have investigated the structure-property relationships for stilbene-based dipolar and quadrupolar chromophores. The emphasis lies on the evolution of the 3PA cross section with the degree of ground-state polarization. For dipolar systems, we find a dominant role played by Deltamu, which expresses the change in dipole moment between the ground state and the 3PA active excited state. Thus, the strategies usually applied to maximize the second-order polarizability beta are also applicable to optimize the 3PA cross section. For quadrupolar systems, the 3PA response is dominated by contributions from channels including various low-lying two-photon allowed states, which limits the applicability of essential-state models. Optimization strategies can be proposed but vary for different ranges of ground-state polarization.
The energetics at the interfaces between metal and monolayers of covalently bound organic molecules is studied theoretically. Despite the molecules under consideration displaying very different frontier orbital energies, the highest occupied molecular levels are found to be pinned at a constant energy offset with respect to the metal Fermi level. In contrast, the molecular properties strongly impact the metal work function. These interfacial phenomena are rationalized in terms of charge fluctuations and electrostatics at the atomic length scale as determined by first-principles calculations.
Three 5,7-pi-extended 8-benzyloxyquinolines, namely 5,7-diphenyl-, 5,7-bis(biphenyl-4-yl)- and 5,7-bis(4-dibenzothiophenyl)-8-benzyloxyquinoline were prepared and investigated as fluorescent pH-probes in nonaqueous solution. Absorption and photoluminescence spectra of the introduced compounds also including the starting material 8-benzyloxy-5,7-dibromoquinoline as well as their N-protonated counterparts were recorded and the results were rationalized by quantum-chemical calculations. A pronounced red shift of the emission occurred upon protonation of the non halogenated derivatives, while the dibromo-derivative is hardly emissive and is virtually not protonated under the conditions used. The diphenyl- and the bis(biphenyl)-derivative especially show promising photoluminescence quantum yields both in the parent and the protonated state making them candidates for the active component in pH sensing applications.
Density functional theory (DFT) calculations based on band structure are used to investigate the electromechanical properties of a molecular junction consisting of a dithiolbenzene molecule sandwiched between two gold slabs. This represents a prototypical system for the field of molecular electronics; such a system has previously been studied in break-junction measurements and electron-transport calculations. The stretching and breaking behavior of the junction is analyzed for different geometric conformations, and it is found that the breakage occurs through dissociation of one of the sulfur-gold bonds with a maximum force of 1.25 nN. The molecular electronic states shift during stretching, and, at the point of highest stress in the junction, the highest occupied molecular orbital (HOMO) of the molecule is located exactly at the Fermi level.
The electron-donor abilities of ten aminophenyl systems and an additional aminothienyl system are compared using density functional theory calculations. The systems studied here include those with amine nitrogen atoms bearing alkyl or aryl groups and those with amine nitrogen atoms as part of a heterocycle. Their abilities to act as donors in electron-transfer processes are assessed from calculated vertical ionization potentials for the aminobenzenes, which are in good agreement with available experimental data. Their abilities to act as intramolecular pi-electron donors in conjugated systems are inferred from the bond lengths and charge densities calculated for the corresponding 4-aminobenzaldehydes and 4-aminobenzonitriles. The computed (13)C NMR chemical shifts for the 4-aminobenzaldehydes and 4-aminobenzonitriles are in good agreement with published and new experimental data. The chemical shifts correlate well with the computed charge densities and can, to some extent, be used as an experimental probe of pi-donor strength. We find that the electron-transfer-donor strengths do not correlate well with pi-donor strengths: these differences can largely be attributed to steric effects.
Three new dipolar chromophores based on a diaklyaminophenyl donor, a pyrrole auxiliary donor, a thiazole auxiliary acceptor, and strong heterocyclic acceptors have been synthesized. For one of these compounds we have measured a very large non-degenerate two-photon cross section of ca. 1500 GM in the near-IR telecommunications range using a pump-probe technique. Calculations indicate the cross section for degenerate two-photon absorption is likely to be ca. 60% of this value.
Electron-phonon coupling in oligo(para-phenylene)s is addressed in terms of the off-resonance Raman intensities of two distinct modes at 1220 and 1280 cm(-1). On the basis of Albrecht's theory, vibrational coupling and Raman intensities are calculated from first-principles quantum-chemical methods. A few-state model is used to evaluate the dependence of the mode intensities on oligomer length, planarity, and excitation wavelength. The link between electron delocalizationconjugation and Raman intensities is highlighted. Extending on prior studies, the present work focuses on providing an in-depth understanding of the origin of this correlation in addition to reproducing experimental findings. The model applied here allows us to interpret the results on a microscopic, quantum-mechanical basis and to relate the observed trends to the molecular orbital structure and nature of the excited states in this class of materials. We find quantitative agreement between the results of the calculations and those of measurements performed on oligo(para-phenylene)s of various chain lengths in the solid state and in solution.
The absorption and emission spectra of most luminescent, pi-conjugated, organic molecules are the mirror image of each other. In some cases, however, this symmetry is severely broken. In the present work, the asymmetry between the absorption and fluorescence spectra in molecular systems consisting of para-linked phenyl rings is studied. The vibronic structure of the emission and absorption bands is calculated from ab initio quantum chemical methods and a subsequent, rigorous Franck-Condon treatment. Good agreement with experiment is achieved. A clear relation can be established between the strongly anharmonic double-well potential for the phenylene ring librations around the long molecular axis and the observed deviation from the mirror image symmetry. Consequences for related compounds and temperature dependent optical measurements are also discussed.
We have applied correlated quantum-chemical methods to investigate the three-photon absorption (3PA) response of a porphyrin triad derivative, where the central macrocycle is linked in mesopositions to two anthracene units via acetylenic bridges. The 3PA frequency-dependent spectrum of this derivative is dominated by a single resonance feature in the transparent region, associated with charge-transfer states between porphyrin and anthracene. The calculations indicate a two order of magnitude enhancement in the 3PA cross section in the triad molecule with respect to the individual entities, which is attributed to close one-, two-, and three-photon resonances together with strong electronic couplings among the units.
Chromophores based on a donor-acceptor-donor structure possessing a large two-photon absorption cross section and one or two mono-aza-15-crown-5 ether moieties, which can bind metal cations, have been synthesized. The influence of Mg(2+) binding on their one- and two-photon spectroscopic properties has been investigated. Upon binding, the two-photon action cross sections at 810 nm decrease by a factor of up to 50 at high Mg(2+) concentrations and this results in a large contrast in the two-photon excited fluorescence signal between the bound and unbound forms, for excitation in the range of 730 to 860 nm. Experimental and computational results indicate that there is a significant reduction of the electron donating strength of the aza-crown nitrogen atom(s) upon metal ion binding and that this leads to a blue shift in the position as well as a reduction in the strength of the lowest-energy two-photon absorption band. The molecules reported here can serve as models for the design of improved two-photon excitable metal-ion sensing fluorophores.
Quantum-chemical calculations of the two-photon absorption (TPA) cross-sections are used to determine the characteristics of the electronic excited states responsible for the observed peaks in the TPA spectra of two bis-dioxaborine-substituted biphenyl derivatives. We find two distinct TPA-active states with very different TPA cross-sections: the difference is explained on the basis of electron correlation. These effects, on the one hand, lead to TPA cross-sections of up to 500 x 10(-50) cm4 s photon(-1) for the state favored by correlation; on the other hand, they limit the overall cross-sections achievable in this class of materials.
Quadrupolar-type substitution of pi-conjugated chromophores with donor and acceptor groups has been shown to increase their two-photon absorption (TPA) response by up to two orders of magnitude. Here, we apply highly correlated quantum-chemical calculations to evaluate the impact of the nature of conjugated bridge and the charge-transfer distance on that enhancement. We compare chromophores with phenylenevinylene-, thienylenevinylene-, polyene-, and indenofluorene-type backbones substituted by dimethylamino and cyano groups. In all compounds, we find a strongly TPA-active A(g) state (either 2A(g) or 3A(g)) in the low-energy region, as well as a higher lying TPA-active state (mA(g)) at close to twice the energy of the lowest lying one-photon allowed state; the smaller energy detuning in the mA(g) states results in very large TPA cross sections delta. We also investigate the influence of the degree of ground-state polarization on TPA. Independent of the nature of the backbone and the donor-acceptor separation, delta displays the same qualitative evolution with a maximum before the cyanine-like limit; the highest TPA cross sections are calculated for distirylbenzene- and polyene-based systems.
The 'lift-out' technique using a focused ion beam microscope was applied to prepare cross-sectional specimens of organic light-emitting diodes for use in transmission electron microscopy. The focused ion beam equally thins the organic/inorganic hybrid devices despite the difference in material hardness of the compounds. This allowed to overcome preparation difficulties of conventional techniques such as ion thinning or ultra-microtomy. Two different samples were prepared and studied by both conventional transmission electron microscopy and analytical electron microscopy to display some of the investigation possibilities which become available with this sample preparation method.