Chuan-Kui Wang

Effects of π centers and symmetry on two-photon absorption cross sections of organic chromophores

We have theoretically examined a series of organic molecules that exhibit large two-photon absorption cross sections in the visible region and that have been synthesized in different laboratories. One- and two-photon absorption cross sections of the four lowest excited states of each molecule have been calculated at the same theoretical level using ab initio response theory. It is found that the molecular length and the one-photon absorption intensity are quite strongly correlated factors, but that a corresponding correlation for the two-photon absorption is much weaker or is missing. In contrast, a most crucial role for large two-photon absorption is played by the pi center. For molecules with a given pi center a symmetrical structure with strong donor groups can result in a maximum two-photon absorption cross section. Our theoretical findings are consistent with some recent experimental observations. The chromophore based on dithienothiophene as pi center attached with symmetrical N,N-diphenylamine donors is found to have the largest two-photon cross section in the visible region among all known one-dimensional two-photon organic materials that have been reported in the literature.

Solvent effects on the electronic structure of a newly synthesized two-photon polymerization initiator

Time-dependent hybrid density functional theory in combination with polarized continuum model has been applied to study the solvent effects on the geometrical and electronic structures, as well as one- and two-photon absorption processes, of a newly synthesized asymmetrical charge-transfer (CT) two-photon absorption (TPA) organic molecule. The TPA cross section calculated from a generalized two-state model and solvatochromic shift of the CT state are found to be solvent dependent, for which a nonmonotonic behavior with respect to the polarity of the solvents has been observed. The calculated properties are in good agreement with the experimental data available. The character of the CT state is visualized by plotting its charge density difference from ground state, in which an excess of electron density on the donor side of the molecule is found. This implies that the excited molecule is ready to donate its electron to the surroundings. The energetic aspect of the electron donation is discussed by examining the solvent dependence of the molecular ground state oxidation potential. The importance of the electron correlation for describing the two-photon absorption is also demonstrated.

Effects of chemical and physical modifications on the electronic transport properties of molecular junctions

We have shown an accurate theoretical description for electronic transport properties of molecular junctions. By applying the elastic scattering Greens function theory approach in combination with the frontier molecular orbital theory for describing the surface-molecule coupling and the hybrid density-functional theory for geometrical and electronic structures calculations, reliable predictions are obtained about the chemical and physical modifications on the current-voltage characteristics of molecular junctions, due to molecular geometrical relaxation, the choice of terminal atoms, the effect of donor/acceptor substitutions, as well as their thermal rotations.

Facile synthesis and systematic investigations of a series of novel bent-shaped two-photon absorption chromophores based on pyrimidine.

The synthesis, structure, and single- and two-photon spectroscopic properties of a series of pyrimidine-based (bent-shaped) molecules are reported. All these stable heterocyclic compounds are fully characterized, and exhibit intense single- and two-photon excited fluorescence (SPEF and TPEF) over a wide spectral range from blue to red, with the spectral peak position of the SPEF being basically the same as that of the TPEF. The well-conjugated pi-systems, observed from the crystal structure, indicate the charge transfer feature of the ground state. Meanwhile, the theoretical and experimental studies indicate that the charge transfer from donor to acceptor is greatly enhanced in the excited states and the different substituted donor groups on the pyrimidine have a large effect on the optical and electrochemical properties. Based on typical structure data and comprehensive spectral data, the following structure-property relationships can be determined: for such bent-shaped chromophores, the absorption and the fluorescence wavelength maximum of the SPEF and TPEF, and two-photon absorption cross sections show a similar trend with increasing electron-donating strength of the corresponding terminal group and the number of branches, while the average bond lengths of the pi-linkage and HOMO-LUMO energy levels show an inverse trend. Experimental data and theoretical calculation provide a coherent picture. With these findings, bent-shaped quadrupolar chromophores combining peak TPA cross sections (up to 2280 GM), broad TPA bands throughout the whole 700-900 nm range, and high fluorescence quantum yields could, thus, be obtained. Such compounds are of particular interest for TPEF microscopy, as well as optical data storage in the visible and NIR regions. A data recording experiment proved the potential application of these materials.

Solvent effects on two-photon absorption of dialkylamino substituted distyrylbenzene chromophore

Solvent effects on the two-photon absorption of a symmetrical diamino substituted distyrylbenzene chromophore have been studied using the density functional response theory in combination with the polarizable continuum model. It is shown that the dielectric medium has a rather small effect both on the bond length alternation and on the one-photon absorption spectrum, but it affects significantly the two-photon absorption cross section. It is found that both one- and two-photon absorptions are extremely sensitive to the planarity of the molecule, and the absorption intensity can be dramatically reduced by the conformation distortion. It has led to the conclusion that the experimentally observed anomalous solvent effect on the two-photon absorption of dialkylamino substituted distyrylbenzene chromophores cannot be attributed to the intrinsic properties of a single molecule and its interaction with solvents.

Current–voltage characteristics of single molecular junction: Dimensionality of metal contacts

Formalisms for current-voltage (I-V) characteristics of a single molecular junction with electrodes possessing one-, two-, and three-dimensional energy distributions are derived and compared in the framework of elastic scattering theory. Model calculations on a device constituted of 1,4 benzene-dithiolate molecule chemically bonded to two gold electrodes are carried out using hybrid density functional theory. It is shown that the dimensionality of metal contacts has a strong effect on the I-V characteristics of the molecular junction, in particular the shape of the conductance curves. Good agreement with experimental I-V curves has been found for the case of the three-dimensional electron system.

Investigations and facile synthesis of a series of novel multi-functional two-photon absorption materials

Six centrosymmetric D–(π–A)3 structural triphenylamine derivatives that can be used as two-photon photopolymerization and optical data storage chromophores, tris[4-(4-pyridylethenyl)phenyl]amine (1), tris[4-(2-pyridylethenyl)phenyl]amine (2), tris(4-cyanoethenylphenyl)amine (3), tris[4-butylacrylatephenyl]amine (4), tris[4-methylacrylatephenyl]amine (5) and tris[4-acrylicethenylphenyl]amine (6), have been successfully synthesized via a triple palladium-catalyzed Heck coupling reaction, and the novel chromophores were fully characterized by elemental analysis, IR, 1H-NMR and ESIMS. The structure for 3 was determined by single crystal X-ray diffraction study. One- and two-photon absorption and fluorescence in various solvents were experimentally investigated. Two-photon initiated polymerization microfabrication and optical data recording experiments were carried out under 780 nm laser radiation, and the possible polymerization mechanism is discussed based on theoretical calculations. All the six chromophores have relatively large two-photon absorption cross-sections, and exhibit optical memory and highly efficient two-photon initiated polymerization abilities.

Solvent dependence of solvatochromic shifts and the first hyperpolarizability of para-nitroaniline: A nonmonotonic behavior

Solvent dependence of the electronic structure and nonlinear optical properties of the para-nitroaniline (pNA) molecule on the polarity of the solutions has been studied using the polarizable continuum model in combination with the hybrid density functional theory. With a supermolecular approach, specific hydrogen bonding effects have also been fully considered. Good agreement between theory and experiments are obtained for both solvatochromic shifts of the charge transfer state and the solvent-induced first hyperpolarizability of pNA.

Synthesis and nonlinear optical properties of novel multi-branched two-photon polymerization initiators

Efficient Witting and Pd-catalyzed Heck coupling methodology are employed to synthesize three multi-branched two-photon photopolymerization initiators diphenyl-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine (abbreviated to DPVPA), phenyl-bis-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine (abbreviated to BPVPA) and tris-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine (abbreviated to TPVPA). The experimental results confirm that all these compounds are good two-photon absorbing chromophores and operative two-photon photopolymerization initiators. The calculated two-photon absorption cross sections of DPVPA, BPVPA and TPVPA for the lowest excited state are 59.3, 30.2 and 42.4 × 10−50 cm4 s photon−1, respectively. A microstructure using BPVPA as initiator has been fabricated under irradiation of 200 fs, from a 76 MHz Ti∶sapphire femtosecond laser at 820 nm. The possible photopolymerization mechanism is discussed.

Inelastic Electron Tunneling Spectroscopy of Gold-Benzenedithiol-Gold Junctions : Accurate Determination of Molecular Conformation

The gold-benzenedithiol-gold junction is the classic prototype of molecular electronics. However, even with the similar experimental setup, it has been difficult to reproduce the measured results because of the lack of basic information about the molecular confirmation inside the junction. We have performed systematic first principles study on the inelastic electron tunneling spectroscopy of this classic junction. By comparing the calculated spectra with four different experimental results, the most possible conformations of the molecule under different experimental conditions have been successfully determined. The relationship between the contact configuration and the resulted spectra is revealed. It demonstrates again that one should always combine the theoretical and experimental inelastic electron tunneling spectra to determine the molecular conformation in a junction. Our simulations have also suggested that in terms of the reproducibility and stability, the electromigrated nanogap technique is much better than the mechanically controllable break junction technique.