Egbert Zojer

Tuning the two-photon absorption response of quadrupolar organic molecules

We apply correlated quantum-chemical techniques to study the origin of the large two-photon absorption (TPA) cross sections, δ, in stilbene derivatives in which electron-donating and electron-withdrawing substituents provide a quadrupolar charge-transfer arrangement. An additional field created by a set of point charges is used to systematically modify the ground-state polarization to determine its consequences for the TPA response. The effect on the molecular structure can be quantified by the evolution of the π-bond-order alternation (π-BOA) of the conjugated backbone. For moderate ground-state polarizations, a marked increase of the TPA response occurs; for large polarizations, δ peaks and then drops dramatically. Insight into the origin of this evolution is gained by comparing the values of δ obtained via the converged sum-over-states approach to the results of a simple three-state model.

Resonant enhancement of two-photon absorption in substituted fluorene molecules

The degenerate and nondegenerate two-photon absorption (2PA) spectra for a symmetric and an asymmetric fluorene derivative were experimentally measured in order to determine the effect of intermediate state resonance enhancement (ISRE) on the 2PA cross section delta. The ability to tune the individual photon energies in the nondegenerate 2PA (ND-2PA) process afforded a quantitative study of the ISRE without modifying the chemical structure of the investigated chromophores. Both molecules exhibited resonant enhancement of the nonlinearity with the asymmetric compound showing as much as a twentyfold increase in delta. Furthermore, the possibility of achieving over a one order of magnitude enhancement of the nonlinearity reveals the potential benefits of utilizing ND-2PA for certain applications. To model ISRE, we have used correlated quantum-chemical methods together with the perturbative sum-over-states (SOS) expression. We find strong qualitative and quantitative correlation between the experimental and theoretical results. Finally, using a simplified three-level model for the SOS expression, we provide intuitive insight into the process of ISRE for ND-2PA.

Bis(dioxaborine) compounds with large two-photon cross sections, and their use in the photodeposition of silver

Compounds in which two dioxaborines are linked by a conjugated bridge exhibit high two-photon cross sections and can be used as sensitisers for the photodeposition of metallic silver lines.

Quantitative prediction of optical excitations in conjugated organic oligomers: A density functional theory study

We present a density functional theory (DFT) study on excitations in conjugated organic materials including oligomers of α-thiophenes, para-phenylenes, para-phenylenevinylenes and ladder-type para-phenylenes. Time-dependent DFT was applied to study optically allowed transitions within the singlet and triplet manifolds, establishing good agreement with experimental data. The splitting between the lowest lying singlet and triplet states is calculated and found to match available experimental data. Additionally, we have employed a Hartree–Fock configuration interaction scheme limited to single excitations for the optimization of excited state geometries in ladder-type para-phenylenes and used time-dependent DFT to calculate the transition energies for fluorescence. Also in this case we found excellent agreement with experimental data.

Twisted π-system electro-optic chromophores. A CIS vs. MRD-CI theoretical investigation

Abstract Twisted, merocyanine-type molecules are investigated theoretically as chromophores, whose optical and electro-optic responses can be controlled via steric repulsion-mediated tuning of the central dihedral angle. Employing multi-reference determinant computational methods results in significant modifications to the quantitative aspects of the response properties, compared to values computed using a single determinant ground state, and leads to computed hyperpolarizabilities even larger than previously estimated. Using either computational approach demonstrates that fine control of the sharp variations in response properties can be achieved by using substituent-induced steric repulsions to modify the internal dihedral twist angles.

Excited state localization in organic molecules consisting of conjugated and nonconjugated segments

We investigate, both experimentally and theoretically, a series of novel molecules consisting of conjugated segments (such as stilbene, naphthylene, and anthrylene) that are separated from each other by nonconjugated bridges. Excited state localization effects are studied theoretically by post-Hartree–Fock calculations—taking into account electron correlation effects. In this context, we compute the electron-hole two-particle wave functions for the prominent excited states and discuss the nature of the molecular orbitals involved in their description. We also investigate geometry relaxation effects following the electronic excitations in order to locate the regions where the strongest rearrangement of the electron density occurs. These conceptionally different approaches (relying also on different semiempirical Hamilton operators and configuration interaction techniques) yield consistent results regarding the localization of the excitations and thus prove helpful to determine the nature of the lowest exci...

Efficient acceptor groups for NLO chromophores : competing inductive and resonance contributions in heterocyclic acceptors derived from 2 -dicyanomethylidene -3 -cyano -4,5,5 -trimethyl -2,5 -dihydrofuran

The 2-dicyanomethylidene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (TCF) group has been identified as a particularly strong acceptor suitable for the realization of highly efficient second-order NLO chromophores. To provide guidelines for a further improvement of heterocyclic acceptors, we present Sum-Over-States (SOS) calculations of second-order polarizabilities (β) for a series of donor–π-bridge–acceptor (D–π–A) chromophores. In particular, we investigate the effect of replacing the oxygen atom of the furan ring in TCF by a variety of other groups, X (SiH2, CH2, CCH2, NH, CO, S, CCHNO2, SO, and SO2). Overall β is found to increase with the inductive electron-withdrawing character of X, with β being further increased or decreased when X has π-accepting or π-donating character. Within the framework of the two-level model, the effects associated with the π-donating and accepting character can be understood in terms of destabilizing and stabilizing effects on the lowest unoccupied molecular orbitals of the dipolar chromophores. When using the stronger 3-methyl-4-cyano-5-dicyanomethylidene-2-oxo-3-pyrroline (TCP) acceptor and replacing the pyrroline nitrogen atom with the same range of X groups, the inductive electron-withdrawing and π-accepting or π-donating characters of X have the same impact on β as in the TCF motif.

Quantum-chemical investigation of second-order nonlinear optical chromophores: Comparison of strong nitrile-based acceptor end groups and role of auxiliary donors and acceptors

We report a detailed quantum-chemical investigation of donor-acceptor substituted dipolar nonlinear optical chromophores incorporating the 4-(dimethylamino)phenyl donor end group and a variety of strong heterocyclic acceptor end groups, including tricyanofurans and tricyanopyrroles. In particular, we study the variation of the molecular second-order polarizability (beta) with the acceptor end group and when inserting auxiliary donors (thiophene) and acceptors (thiazole) into the pi bridge. Both finite-field calculations (in the context of local contributions) and sum-over-states calculations were carried out in order to probe the relationship between beta and the chemical structure of the various chromophores. The trends obtained with these two methods are fully consistent. The large beta values (up to 700 x 10(-30) esu) as well as the observed tunability of the optical absorption maximum (lambda(max)) make the chromophores investigated here interesting candidates for use in electro-optic applications at telecommunications wavelengths.

On the polarization of the green emission of polyfluorenes

An experimental and theoretical study of the anisotropic optical properties of polyfluorenes (PFs) bearing ketonic defects is presented. Polarized emission experiments performed on photooxidized aligned PF layers indicate that the transition dipole of the “green” CT π-π* transition of the keto-defect is oriented parallel to the chain direction. It is further observed that the polarization ratio of the green emission is slightly smaller than that of the blue emission component originating from undisturbed chains. Quantum mechanical calculations have been performed to support these observations. It is shown that the transition dipole moment of the CT π-π* transition of the defect is slightly misaligned with respect to the π-π* transition of the undisturbed PF chain, and that the angle between both depends on the chain conformation. For the most probably 5/2 helical conformation, this angle is, however, smaller than 5°. Further, polarized PL spectroscopy with polarized excitation has been performed to determ...

Effective conjugation and Raman intensities in oligo(para-phenylene)s: A microscopic view from first-principles calculations

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 Albrechts 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.