Alexander L. Burin

Elementary steps for charge transport in DNA: thermal activation vs. tunneling

Abstract Using stacks of Watson–Crick base pairs as an important example of multichromophoric molecular assemblies, we studied charge migration in DNA with special emphasis on the mechanism of hole hopping between neighboring guanines (G) connected by the adenine–thymine (AT) bridge. The tight-binding model proposed for this elementary step shows that for short AT bridges, hole transfer between two G bases proceeds via quantum mechanical tunneling. By contrast, hopping over long bridges requires thermal activation. The condition for crossover between tunneling and thermal activation near room temperature is specified and applies to the analysis of experimental data. We show that thermal activation dominates, if the bridge between two G bases contains more than three AT pairs. Our theoretical findings predict that the replacement of AT base pairs by GC pairs increases the efficiency of hole transport only in the case of short base pair sequences. For long sequences, however, the opposite effect is expected.

Random lasers with coherent feedback

We review our recent work on lasing in active random media. Light scattering, which had been regarded as detrimental to lasing action for a long time, actually provided coherent feedback for lasing. The fundamental difference and transition between a random laser with coherent feedback and a random laser with incoherent feedback were illustrated. We also trapped laser light in micrometer-sized random media. The trapping was caused by disorder-induced scattering and interference. This nontraditional way of light confinement has important applications to microlasers.

Direct Measurement of the Dynamics of Hole Hopping in Extended DNA G-Tracts. An Unbiased Random Walk

We report the measurement of distance- and temperature-dependent rate constants for charge separation in capped hairpins in which a stilbene hole acceptor and hole donor are separated by A(3)G(n) diblock polypurine sequences consisting of 3 adenines and 1-19 guanines. The longer diblock systems obey the simplest model for an unbiased random walk, providing a direct measurement of k(hop) = 4.3 × 10(9) s(-1) for a single reversible G-to-G hole hopping step, somewhat faster than the value of 1.2 × 10(9) s(-1) calculated for A-tract hole hopping. The temperature dependence for hopping in A(3)G(13) provides values of E(act) = 2.8 kcal/mol and A = 7 × 10(9) s(-1), consistent with a weakly activated, conformationally gated process.

Spin effects on the luminescence yield of organic light emitting diodes

The influence of the excitation spectrum on the quantum yield is investigated for organic molecules used in light emitting diodes (LEDs). The significance of the competition between radiative and nonradiative recombination channels is pointed out. A system of master equations is proposed to describe the relaxation of lower levels of the excited molecule, and the solution for the fluorescence yield is obtained assuming relatively weak vibronic coupling. The results are used to interpret the experimental data in oligothiophenes, and general approaches are proposed to increase the relative weight of the radiative decay channel and correspondingly enhance the working properties of organic light emitting diodes. In particular, the fluorescence quantum yield can exceed the simple estimate of 0.25.

Structure Dependent Energy Transport: Relaxation-Assisted 2DIR Measurements and Theoretical Studies

Vibrational energy relaxation and transport in a molecule that is far from thermal equilibrium can affect its chemical reactivity. Understanding the energy transport dynamics in such molecules is also important for measuring molecular structural constraints via relaxation-assisted two-dimensional infrared (RA 2DIR) spectroscopy. In this paper we investigated vibrational relaxation and energy transport in the ortho, meta, and para isomers of acetylbenzonitrile (AcPhCN) originated from excitation of the CN stretching mode. The amplitude of the cross-peak among the CN and CO stretching modes served as an indicator for the energy transport from the CN group toward the CO group. A surprisingly large difference is observed in both the lifetimes of the CN mode and in the energy transport rates for the three isomers. The anharmonic DFT calculations and energy transport modeling performed to understand the origin of the differences and to identify the main cross-peak contributors in these isomers described well the majority of the experimental results including mode excited-state lifetimes and the energy transport dynamics. The strong dependence of the energy transport on molecular structure found in this work could be useful for recognizing different isomers of various compounds via RA 2DIR spectroscopy.

Two-photon pumping of a random laser

Random lasing originates from pumping disordered materials by high-intensity and high-frequency light. The performance of random lasers is restricted due to the absorption of pumping light within the interfacial layer. The lifetime of optical modes there is short due to the emission of photons to the outside, and it is hard to pump them sufficiently for lasing. The opportunity to excite the modes far from the material surface using two-photon absorption is investigated within the diffusion model. The authors show that lasing requires lower pump power for the two-photon pumping mechanism under conditions of negligible absorption of the emitted light. Experimental implications and restrictions are discussed.

Ballistic Energy Transport in Oligomers

The development of nanocomposite materials with desired heat management properties, including nanowires, layered semiconductor structures, and self-assembled monolayer (SAM) junctions, attracts broad interest. Such materials often involve polymeric/oligomeric components and can feature high or low thermal conductivity, depending on their design. For example, in SAM junctions made of alkane chains sandwiched between metal layers, the thermal conductivity can be very low, whereas the fibers of ordered polyethylene chains feature high thermal conductivity, exceeding that of many pure metals. The thermal conductivity of nanostructured materials is determined by the energy transport between and within each component of the material, which all need to be understood for optimizing the properties. For example, in the SAM junctions, the energy transport across the metal-chain interface as well as the transport through the chains both determine the overall heat conductivity, however, to separate these contributions is difficult. Recently developed relaxation-assisted two-dimensional infrared (RA 2DIR) spectroscopy is capable of studying energy transport in individual molecules in the time domain. The transport in a molecule is initiated by exciting an IR-active group (a tag); the method records the influence of the excess energy on another mode in the molecule (a reporter). The energy transport time can be measured for different reporters, and the transport speed through the molecule is evaluated. Various molecules were interrogated by RA 2DIR: in molecules without repeating units (disordered), the transport mechanism was expected and found to be diffusive. The transport via an oligomer backbone can potentially be ballistic, as the chain offers delocalized vibrational states. Indeed, the transport regime via three tested types of oligomers, alkanes, polyethyleneglycols, and perfluoroalkanes was found to be ballistic, whereas the transport within the end groups was diffusive. Interestingly, the transport speeds via these chains were different. Moreover, the transport speed was found to be dependent on the vibrational mode initiating the transport. For the difference in the transport speeds to be explained, the chain bands involved in the wavepacket formation were analyzed, and specific optical bands of the chain were identified as the energy transporters. For example, the transport initiated in alkanes by the stretching mode of the azido end group (2100 cm(-1)) occurs predominantly via the CH2 twisting and wagging chain bands, but the transport initiated by the C=O stretching modes of the carboxylic acid or succinimide ester end groups occurs via C-C stretching and CH2 rocking bands of the alkane chain. Direct formation of the wavepacket within the CH2 twisting and wagging chain bands occurs when the transport is initiated by the N═N stretching mode (1270 cm-1) of the azido end-group. The transport via optical chain bands in oligomers involves rather large vibrational quanta (700-1400 cm(-1)), resulting in efficient energy delivery to substantial distances. Achieved quantitative description of various energy transport steps in oligomers, including the specific contributions of different chain bands, can result in a better understanding of the transport steps in nanocomposite materials, including SAM junctions, and lead towards designing systems for molecular electronics with a controllable energy transport speed.

Stability of Bose-Einstein condensates of hot magnons in yttrium iron garnet films.

We investigate the stability of the recently discovered room-temperature Bose-Einstein condensate (BEC) of magnons in yttrium iron garnet films. We show that magnon-magnon interactions depend strongly on the external field orientation, and that the BEC in current experiments is actually metastable-it only survives because of finite-size effects, and because the BEC density is very low. On the other hand a strong field applied perpendicular to the sample plane leads to a repulsive magnon-magnon interaction; we predict that a high-density room-temperature magnon BEC should then form in this perpendicular field geometry.

Temperature and field dependence of the charge injection from metal electrodes into random organic media

We consider the average injection rate from a metal electrode into disordered organic media, and its field dependence for a fixed density of carriers in the metal. At sufficiently high field the injection occurs through particular pathways, lowering the energetic barrier between the metal Fermi energy and the molecular levels of the organic layer. Assuming a Gaussian distribution of random energies, we find the dependence of current j on field E j∝exp(βE) in the relevant domain of electric fields. This behavior agrees both with numerical simulations of three-dimensional systems and with experimental data.

Physics of Proteins at Low Temperature

We present results of a hole burning study with thermal cycling and waiting time spectral diffusion experiments on a modified cytochrome - c protein in its native as well as in its denatured state. The experiments show features which seem to be characteristic for the protein state of matter and its associated dynamics at low temperature. The properties responsible for the observed patterns are organisation paired with randomness and, in addition, the finite size which gives rise to surface and solvent effects. We discuss some general model approaches which might serve as guide lines for understanding these features.