Boiko Cohen

Base stacking controls excited-state dynamics in A-T DNA

Solar ultraviolet light creates excited electronic states in DNA that can decay to mutagenic photoproducts. This vulnerability is compensated for in all organisms by enzymatic repair of photodamaged DNA. As repair is energetically costly, DNA is intrinsically photostable. Single bases eliminate electronic energy non-radiatively on a subpicosecond timescale, but base stacking and base pairing mediate the decay of excess electronic energy in the double helix in poorly understood ways. In the past, considerable attention has been paid to excited base pairs. Recent reports have suggested that light-triggered motion of a proton in one of the hydrogen bonds of an isolated base pair initiates non-radiative decay to the electronic ground state. Here we show that vertical base stacking, and not base pairing, determines the fate of excited singlet electronic states in single- and double-stranded oligonucleotides composed of adenine (A) and thymine (T) bases. Intrastrand excimer states with lifetimes of 50–150 ps are formed in high yields whenever A is stacked with itself or with T. Excimers limit excitation energy to one strand at a time in the B-form double helix, enabling repair using the undamaged strand as a template.

Mechanism of Charge Transfer and Recombination Dynamics in Organo Metal Halide Perovskites and Organic Electrodes, PCBM, and Spiro-OMeTAD: Role of Dark Carriers.

Despite the unprecedented interest in organic-inorganic metal halide perovskite solar cells, quantitative information on the charge transfer dynamics into selective electrodes is still lacking. In this paper, we report the time scales and mechanisms of electron and hole injection and recombination dynamics at organic PCBM and Spiro-OMeTAD electrode interfaces. On the one hand, hole transfer is complete on the subpicosecond time scale in MAPbI3/Spiro-OMeTAD, and its recombination rate is similar to that in neat MAPbI3. This was found to be due to a high concentration of dark charges, i.e., holes brought about by unintentional p-type doping of MAPbI3. Hence, the total concentration of holes in the perovskite is hardly affected by optical excitation, which manifested as similar decay kinetics. On the other hand, the decay of the photoinduced conductivity in MAPbI3/PCBM is on the time scale of hundreds of picoseconds to several nanoseconds, due to electron injection into PCBM and electron-hole recombination at the interface occurring at similar rates. These results highlight the importance of understanding the role of dark carriers in deconvoluting the complex photophysical processes in these materials. Moreover, optimizing the preparation processes wherein undesired doping is minimized could prompt the use of organic molecules as a more viable electrode substitute for perovskite solar cell devices.

Strickler–Berg analysis of excited singlet state dynamics in DNA and RNA nucleosides

The excited singlet state lifetime of the ribonucleoside uridine was found to be 210+/-30 fs by femtosecond transient absorption spectroscopy. This value is considerably shorter than all previous time-domain measurements. This result and our previous lifetime measurements [see J.-M. L. Pecourt, J. Peon and B. Kohler, J. Am. Chem. Soc., 2001, 123, 10 370] for the other common nucleosides are compared with lifetimes calculated from available photophysical data using the Strickler-Berg (SB) equation. The calculated lifetimes for pyrimidine nucleosides are 10-25% lower than the lifetimes measured in femtosecond transient absorption experiments. For the purine nucleosides, guanosine and adenosine, consideration of just the lowest 1pi --> pi* transition led to predicted lifetimes that are three times greater than experimental ones. On the other hand, inclusion of both of the lowest energy 1pi --> pi* absorption bands in the SB equation resulted in much better agreement with the experimental values. This suggests that both 1pi pi* states of the purine nucleosides contribute to their emission. Decay by the bright 1pi pi* state (or states, in the case of the purines) is believed to be responsible for the experimentally observed lifetimes.

Molecular spectroscopy: Complexity of excited-state dynamics in DNA (Reply)

We have shown that long-lived excited electronic states known as excimers, which arise from base stacking, are formed in high yields in a variety of synthetic DNA oligonucleotides. Markovitsi et al. question our interpretation, and claim that these states can be accounted for by their exciton theory. However, neither this nor their emission data contradict our finding that in single- and double-stranded A·T (adenine–thymine-paired) DNA, excited states decay through long-lived intermediate states in which excitation is shared by stacked bases.

Unraveling Charge Carriers Generation, Diffusion, and Recombination in Formamidinium Lead Triiodide Perovskite Polycrystalline Thin Film

We report on studies of the formamidinium lead triiodide (FAPbI3) perovskite film using time-resolved terahertz (THz) spectroscopy (TRTS) and flash photolysis to explore charge carriers generation, migration, and recombination. The TRTS results show that upon femtosecond excitation above the absorption edge, the initial high photoconductivity (∼75 cm(2) V(-1) s(-1)) remains constant at least up to 8 ns, which corresponds to a diffusion length of 25 μm. Pumping below the absorption edge results in a mobility of 40 cm(2) V(-1) s(-1) suggesting lower mobility of charge carriers located at the bottom of the conduction band or shallow sub-bandgap states. Furthermore, analysis of the THz kinetics reveals rising components of <1 and 20 ps, reflecting dissociation of excitons having different binding energies. Flash photolysis experiments indicate that trapped charge carriers persist for milliseconds.

Femtosecond to millisecond studies of electron transfer processes in a donor–(π-spacer)–acceptor series of organic dyes for solar cells interacting with titania nanoparticles and ordered nanotube array films

Time-resolved emission and absorption spectroscopy are used to study the photoinduced dynamics of forward and back electron transfer processes taking place between a recently synthesized series of donor-(π-spacer)-acceptor organic dyes and semiconductor films. Results are obtained for vertically oriented titania nanotube arrays (inner diameters 36 nm and 70 nm), standard titania nanoparticles (25 nm diameter) and, as a reference, alumina nanoparticle (13 nm diameter) films. The studied dyes contain a triphenylamine group as an electron donor, cyanoacrylic acid part as an electron acceptor, and differ by the substituents in a spacer group that causes a shift of its absorption spectra. Despite a red-shift of the dye absorption band resulting in an improved response to the solar spectrum, smaller electron injection rates and smaller extinction coefficients result in reduced dye sensitized solar cell (DSSC) conversion efficiencies. For the most efficient dye, TPC1, electron injection from the hot locally excited state to titania on a time scale of about 100 fs is suggested, while from the relaxed charge transfer state it proceeds in a non-exponential way with time constants from 1 ps to 50 ps. Our results imply that the latter process involves the trap states below the conduction band edge (or the sub-bandgap tail of the acceptor states), localized close to the dye radical cation, and is accompanied by fast electron recombination to the parent dyes ground state. This process should limit the efficiency of DSSCs made using these types of organic dyes. The residual, slower recombination can be described by a stretched exponential decay with a characteristic time of 0.5 μs and a dispersion parameter of 0.33. Both the electron injection and back electron transfer dynamics are similar in titania nanoparticles and nanotubes. Variations between the two film types are only found in the time resolved emission transients, which are explained in terms of the difference in local electric fields affecting the position of the emission bands.

Mapping the distribution of an individual chromophore interacting with silica-based nanomaterials.

Exploring the interactions of molecules with silica-based mesoporous and nanoparticle materials at the atomic level and understanding of the forces that govern such H-bonds and electrostatic interactions are of fundamental importance to nanocatalysis, nanomedicine, and nanophotonics. In our approach, we studied in single-molecule time and spectral domains a proton-transfer chromophore complexed (by diffusion) and covalently bonded to MCM-41 mesoporous nanomaterial and silica particles. The results reveal strong dependence of the distribution and behavior of the interacting single molecule with the nanopores on the mode of sample preparation and nature of the involved interaction. The change at the single molecule level results in an up to 126 nm (approximately 4650 cm(-1)) spectral shift (from 462 to 588 nm) and almost two times longer lifetime. Furthermore, a change in the electronic charges of the mesoporous framework results in significant narrowing in the emission band of the guest. The results are explained in terms of electronic nanoconfinement but at a single-molecular level.

Ultrafast Photodynamics of Drugs in Nanocavities: Cyclodextrins and Human Serum Albumin Protein

In this feature article, we discuss recent advances in studying ultrafast dynamic and structural aspects of host-guest interactions. Steady-state and time-resolved techniques exploring events from the femto- to nanosecond regime were used to examine the ultrafast photodynamics and subsequent events in selected nanostructures of the formed complexes. These consist of aromatic systems, biologically relevant molecules, and drugs trapped within cyclodextrins (CD) and human serum albumin (HSA) protein pockets. We examine the effects exerted by these chemical and biological cavitands on internal twisting motions, proton transfer and charge transfer, and cis-trans isomerization reactions that may occur in the confined molecular systems. In addition, the influence of a restricting environment on the interaction of guest molecules with biological water is considered. The dynamic details of the complexes (diffusion, early interactions, formation, stability, internal guest diffusion, and conformational changes) and the excited-state relaxation pathways, rate constants of the involved processes, and changes in the electronic distribution within encapsulated guests gave clues to elucidate their photobehavior and are relevant to the photostability and delivery of drugs when using nanocarriers.

Exploring the Ground and Excited States Structural Diversity of Levosimendan, a Cardiovascular Calcium Sensitizer †

Exploring the relationship between the structure and dynamics of a molecular system is fundamental to a better understanding of its function. Here, we report on studies of femtosecond dynamics of the most stable molecular structures of a cardiovascular drug, levosimendan (LSM), in water at three different pHs, in chemical (β-cyclodextrin, β-CD) and biological (human serum albumin protein, HSA) nanocavities, and in two organic solvents with different viscosities. In the used organic solvents, the structural dynamics, ranging from 50 fs to 3 ps, depends on the viscosity of the solvent, reflecting the involvement of a twisting motion in the excited molecule. In water solutions at pH 3 and 5, the excited neutral form is decaying in a time of ∼0.4 ps, undergoing an ultrafast (<50 fs) intramolecular charge transfer (ICT) to generate charge transfer species decaying in ∼1 ps. In neutral (pH 7) and alkaline water (pH 12), the LSM is present in its anion structure at the ground state. In these media, the experiments reveal, in addition to the ultrafast decay of the anionic structure (1.3 ps), the formation of an ICT state having (n, π*) character, produced in ∼0.3 ps and decaying in ∼0.5 ps. Encapsulation by β-CD and HSA protein leads to a 1:1 stoichiometry complex, which shows longer decaying times (4 and 7 ps, respectively) of the caged anionic forms due to the nanoconfinement. Our results show a structural diversity of the LSM dynamics, reflecting its intimate interaction with its surrounding. We believe that the reported findings and the related discussion and conclusions bring new knowledge for a better understanding of the molecular activity of this drug, taking into account its rich structural dynamics. Furthermore, the results might be relevant for a better drug design and nanodelivery involving CDs and proteins.

Femtosecond to second studies of a water-soluble porphyrin derivative in chemical and biological nanocavities.

The interactions of 5,10,15,20-tetrakis(4-sulfonatophenyl)-porphyrin (TSPP) with a quaternary ammonium modified β-cyclodextrin (QA-β-CD) and human serum albumin (HSA) protein in aqueous solutions at pH 7 were studied using steady-state, stopped-flow, and femtosecond to millisecond spectroscopy. TSPP forms 1:1 and 1:2 complexes with QA-β-CD (K(1) = 1.9 × 10(5) M(-1) and K(2) = 7 × 10(3) M(-1)) at 293 K, whereas with the HSA protein only 1:1 complex (K(1) = 1.7 × 10(6) M(-1)) has been found. The chemical and biological nanocavities have notable effects on the fluorescence lifetimes of the Q(x) state (from 9.3 to 11.1 ns in QA-β-CD and 11.6 ns in HSA). Furthermore, the rotational times (400 ps for the free TSPP, 1.6 and 19 ns for QA-β-CD and HSA protein complexes, respectively) clearly indicate the robustness of the formed entities. The confined environment does not affect much the fs dynamics (0.1-0.2 ps) of the encapsulated molecule. However, it clearly affect the ps one (1-2 ps (H(2)O) and 5-10 ps (QA-β-CD and HSA)). The effect of O(2) on the relaxation of the triplet state of the free and encapsulated TSPP is also studied and the obtained results are discussed in light of the shielding effect provided by the chemical and biological cavities. The observed difference, longer triplet lifetime upon encapsulation, might be relevant to the efficiency of this porphyrin in photodynamic therapy. The presteady-state kinetics of the TSPP:HSA has been studied by the stopped-flow spectrometer, and a two-step model was proposed for the complexation processes. The results show the importance of the initial association step for the overall ligand recognition process. This first step occurs with rate constant of ~4 × 10(5) M(-1) s(-1), which is about 5 orders of magnitude larger than the rate constant of the consecutive relaxation processes. We believe that our observations of molecular interaction between TSPP, QA-β-CD, and HSA protein from femtosecond to second at both ground and electronically first excited state give detailed information to improve our understanding of this kind of system and thus for a better design of drug delivery nanocarriers.