Ivan A. Mikhailov

DFT-Based Methods in the Design of Two-Photon Operated Molecular Switches

Conjugated organic molecules with photochromic properties are being extensively studied as prospective optical switching and data storage materials. Among different photochromic compounds, diarylethenes demonstrate thermal stability, fatigue resistance, and high quantum yield. The mechanism of photoswitching in diarylethenes involves a symmetry-allowed conrotatory electrocyclic reaction, initiated by UV light. Replacement of one UV photon with two near-IR ones would offer a number of practical advantages, including drastic increase in storage capacity via three-dimensional multilayer design. For this purpose we designed a prototype molecule with a two-photon absorbing (2PA) pendant substituent, attached to the photochromic diarylethene moiety. However, this molecule was experimentally shown to have lost the photoswitching properties. We analyze reasons for this loss using quantum chemistry tools. Analysis of the nodal structure of the frontier Kohn-Sham orbitals, allowed us to trace the route of the problem to the lone pair orbital of the 2PA substituent falling within the HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap of the photoreactive diarylethene moiety. We suggest a chemical modification of the 2PA substituent in order to restore the order of the orbitals. Potential energy plots along the reaction coordinate at the M05-2X/6-31G* theory level for the prototype 2PA photochromic molecule before and after the modification confirm the predictive capability of the proposed orbital approach. The Slater transition state method was used to obtain geometries along the reaction pathway by the constrained optimization of excited states, whereas potential energy curves were plotted using the recently proposed (Mikhailov, I. A.; Tafur, S.; Masunov, A. E. Phys. Rev. A 2008, 77 (1), 012510) a posteriori Tamm-Dancoff approximation to the time-dependent density functional theory in second order of the external field. We show that this combination is able to produce accurate potential surfaces for 1B and 2A excited states, as compared to available experimental data and results of high-level multireference wave function theory methods.

Fluorene-based metal-ion sensing probe with high sensitivity to Zn2+ and efficient two-photon absorption.

The photophysical, photochemical, two-photon absorption (2PA) and metal ion sensing properties of a new fluorene derivative (E)-1-(7-(4-(benzo[d]thiazol-2-yl)styryl)-9,9-bis(2-(2-ethoxyethoxy)ethyl)-9H-fluoren-2-yl)-3-(2-(9,10,16,17,18,19,21,22,23,24-decahydro-6H dibenzo[h,s][1,4,7,11,14,17]trioxatriazacycloicosin-20(7H)-yl)ethyl)thiourea (1) were investigated in organic and aqueous media. High sensitivity and selectivity of 1 to Zn(2+) in tetrahydrofuran and a water/acetonitrile mixture were shown by both absorption and fluorescence titration. The observed complexation processes corresponded to 1:1 stoichiometry with the range of binding constants approximately (2-3) x 10(5) M(-1). The degenerate 2PA spectra of 1 and 1/Zn(2+) complex were obtained in the 640-900 nm spectral range with the maximum values of two-photon action cross section for ligand/metal complex approximately (90-130) GM, using a standard two-photon induced fluorescence methodology under femtosecond excitation. The nature of the 2PA bands was analyzed by quantum chemical methods and a specific dependence on metal ion binding processes was shown. Ratiometric fluorescence detection (420/650 nm) provided a good dynamic range (10(-4) to 10(-6) M) for detecting Zn(2+), which along with the good photostability and 2PA properties of probe 1 makes it a good candidate in two-photon fluorescence microscopy imaging and sensing of Zn ions.

Recombination of H+3 ions in the afterglow of a He–Ar–H2 plasma

Recombination of H+3 with electrons is studied in a low-temperature plasma containing He, H2 and Ar at different He and H2 densities. The effective plasma recombination rate is driven by binary, H+3 + e−, and ternary, H+3 + e−+ He, processes with the rate coefficients 7.5 × 10−8 cm3 s−1 and 2.8 × 10−25 cm6 s−1 respectively at 260 K. We suggest that the ternary recombination involves formation of neutral highly excited Rydberg H3 followed by an l-changing collision with He. The difference between recombination of para- and ortho-H+3 is discussed.

Steady-state and femtosecond transient absorption spectroscopy of new two-photon absorbing fluorene-containing quinolizinium cation membrane probes.

The synthesis, linear photophysical characterization, and nonlinear optical properties of two new symmetrical fluorene-containing quinolizinium derivatives, 2,8-bis((E)-2-(7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl)vinyl)quinolizinium hexafluorophosphate (1) and 2,8-bis((E)-2-(7-((7-(diphenylamino)-9,9-dihexyl-9H-fluoren-2-yl)ethynyl)-9,9-dihexyl-9H-fluoren-2yl)vinyl)quinolizinium hexafluorophosphate (2), are reported. The nature of the dual-band steady-state fluorescence emission of 1 and 2 was determined, and violation of Kashas rule along with a strong dependence on solvent polarity were shown. A relatively complex structure of two-photon absorption (2PA) spectra of 1 and 2, with maximum cross sections of ∼400-600 GM, was determined using the open aperture Z-scan method. Different types of fast relaxation processes with characteristic times of 0.3-0.5 ps and 1.5-2 ps were observed in the excited states of the new compounds via femtosecond transient absorption pump-probe spectroscopy. To better understand the photophysical behavior of 1 and 2, a quantum-mechanical study was undertaken using TD-DFT and ZINDO/S methods. Simulated linear absorption spectra were found to be in good agreement with experimental data, while 2PA cross sections were overestimated. Although the new dyes were highly fluorescent in nonpolar solvents, they were essentially nonfluorescent in polar media. Significantly, the quinolizinium dyes exhibited fluorescence turn-on behavior upon binding to bovine serum album (BSA) protein, exhibiting over 4-fold fluorescence enhancement, which was a finding that was leveraged to demonstrate cell membrane fluorescence imaging of HeLa cells.

Superfluorescent Squaraine with Efficient Two-Photon Absorption and High Photostability

The synthesis, linear photophysical, two-photon absorption (2PA), femtosecond transient absorption, and superfluorescence properties of a new symmetrical squaraine derivative (1) are reported. Steady-state linear spectral and photochemical properties, fluorescence lifetimes, and excitation anisotropy of 1 were investigated in various organic solvents. High fluorescence quantum yields (≈0.7) and very high photostability (photodecomposition quantum yields ≈10(-6)-10(-8)) were observed. An open-aperture Z-scan method was used to obtain 2PA spectra of 1 over a broad spectral range (maximum 2PA cross section ≈1000 GM). Excited-state absorption (ESA) and gain was observed by femtosecond transient absorption spectroscopy, in which both reached a maximum at approximately 500 fs. Squaraine 1 exhibits efficient superfluorescence. The quantum chemical study of 1 revealed the simulated vibronic nature of the 1PA and 2PA spectra were in good agreement with experimental data; this may provide the ability to predict potential advanced photonic materials.

Are density functional theory predictions of the Raman spectra accurate enough to distinguish conformational transitions during amyloid formation

We report density functional theory (DFT) calculations of the Raman spectra for hexapepetides of glutamic acid and lysine in three different conformations (α, β and PPII). The wave numbers of amide I, amide II and amide III bands of all three conformations predicted at B3LYP/6-31G and B3LYP/6-31G* are in good agreement with previously reported experimental values of polyglutamic acid and polylysine. Agreement with experiment improves when polarization functions are included in the basis set. Explicit water molecules, H-bonded to the backbone amide groups were found to be absolutely necessary to obtain this agreement. Our results indicate that DFT is a promising tool for assignment of the spectral data on kinetics of conformational changes for peptides during amyloid formation.

The role of the Jahn-Teller coupling in dissociative recombination of H3O+ and H ions

We discuss the importance of the non Born-Oppenheimer Jahn-Teller coupling in the dissociative recombination of H3O+ and H ions with low energy electrons. The coupling is known to be important for dissociative recombination of H, for which detailed and simplified theoretical models have been previously developed. In this study we discuss how the detailed theoretical model reproduces individual resonances in the experimental spectrum of H. We present a simplified theoretical model of the dissociative recombination of H3O+. Similarly to H, we consider the Jahn-Teller coupling as a key factor responsible for the capture of the electron into a Rydberg state associated with excited vibrational levels of the ion. We use a simplified Jahn-Teller model for highly symmetrical ions (not only for H3O+ and H) and adopt the normal mode approximation for the vibrational states of H3O+. Using the multi-channel quantum defect formalism and accurate ab initio calculations, we derive the cross sections for electron capture by the ion that is initially in the ground vibrational level. In our approximation, once the electron is captured by the ion, the autoionization probability is neglected compared to the predissociation probability. This allows us to evaluate the cross section for the dissociative recombination of H3O+ using just a few parameters obtained from ab initio calculation. The total cross section obtained is in good agreement with data from storage ring experiments.

Theoretical Photochemistry of the Photochromic Molecules Based on Density Functional Theory Methods

Mechanism of photoswitching in diarylethenes involves the light-initiated symmetry-allowed disrotatory electrocyclic reaction. Here we propose a computationally inexpensive Density Functional Theory (DFT) based method that is able to produce accurate potential surfaces for the excited states. The method includes constrained optimization of the geometry for the ground and two excited singlet states along the ring-closing reaction coordinate using the Slater Transition State method, followed by single-point energy evaluation. The ground state energy is calculated with the broken-symmetry unrestricted Kohn-Sham formalism (UDFT). The first excited state energy is obtained by adding the UDFT ground state energy to the excitation energy of the pure singlet obtained in the linear response Time-Dependent (TD) DFT restricted Kohn-Sham formalism. The excitation energy of the double excited state is calculated using a recently proposed (Mikhailov, I. A.; Tafur, S.; Masunov, A. E. Phys. Rev. A 77, 012510, 2008) a posteriori Tamm-Dancoff approximation to the second order response TD-DFT.

Predictions of Two Photon Absorption Profiles Using Time-Dependent Density Functional Theory Combined with SOS and CEO Formalisms

Two-photon absorption (2PA) and subsequent processes may be localized in space with a tightly focused laser beam. This property is used in a wide range of applications, including three dimensional data storage. We report theoretical studies of 5 conjugated chromophores experimentally shown to have large 2PA cross-sections. We use the Time Dependent Density Functional Theory (TD-DFT) to describe the electronic structure. The third order coupled electronic oscillator formalism is applied to calculate frequency-dependent second order hyperpolarizability. Alternatively, the sum over states formalism using state-to-state transition dipoles provided by the a posteriori Tamm-Dancoff approximation is employed. It provides new venues for qualitative interpretation and rational design of 2PA chromophores.