6. Rehybridization of the Acceptor (RICT) 3908 7. Planarization of the Molecule (PICT) 3909 III. Fluorescence Spectroscopy 3909 A. Solvent Effects and the Model Compounds 3909 1. Solvent Effects on the Spectra 3909 2. Steric Effects and Model Compounds 3911 3. Bandwidths 3913 4. Isoemissive Points 3914 B. Dipole Moments 3915 C. Radiative Rates and Transition Moments 3916 1. Quantum Yields and Radiationless Deactivation Processes 3916

Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures

About Supramolecular Assemblies of π-Conjugated Systems

Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

Charge-Transfer and Energy-Transfer Processes in π-Conjugated Oligomers and Polymers: A Molecular Picture

Graphenes as potential material for electronics.

The room-temperature fluorescence properties of DNA nucleoside and nucleotide aqueous solutions are studied by steady-state and time-resolved spectroscopy. The steady-state fluorescence spectra, although peaking in the near-UV region, are very broad, extending over the whole visible domain. Quantum yields are found to be mostly higher and the fluorescence decays faster than those reported in the literature. The fluorescence spectra of the 2‘-deoxynucleosides are identical to those of the 2‘-deoxynucleotides, with the exception of 2‘-deoxyadenosine, for which a difference in the spectral width is observed. The steady-state absorption and fluorescence spectra do not show any concentration dependence in the range 5 × 10-6 to 2 × 10-3 M. All fluorescence decays are complex and cannot be described by monoexponential functions. From the zero-time fluorescence anisotropies recorded at 330 nm, it is deduced that after excitation at 267 nm the largest modification in the electronic structure is exhibited by 2‘-deo...

Fluorescence Properties of DNA Nucleosides and Nucleotides: A Refined Steady-State and Femtosecond Investigation

The spectroscopic properties of discotic hexa-alkylthiotriphenylenes are studied in solution and thin films and compared to those of hexa-alkyloxytriphenylenes. The solution properties are analyzed in the light of CS-INDO-CIPSI quantum chemistry calculations. The absorption maximum is assigned to the degenerate S0 → S4 transition. The fluorescence of the neat phases is attributed to weakly bound excimers. The phase transition leading from ordered to disordered columnar stacks induces an increase in the oscillator strength of the S0 → S1 transition and favors excimer formation. The influence of structural disorder on the properties of the delocalized states is rationalized by using various approximations within the frame of the exciton theory; three models for the calculation of the exciton coupling (point dipole, extended dipole, atomic transition charge distribution) are tested, short and long range interactions are considered, and the introduction of a dielectric constant is discussed. The best agreemen...

Influence of disorder on electronic excited states : an experimental and numerical study of alkylthiotriphenylene columnar phases

Absorption of ultraviolet light by DNA is known to lead to carcinogenic mutations, but the processes between photon absorption and the photochemical reactions are poorly understood. In their study of the excited-stated dynamics of model DNA helices using femtosecond transient absorption spectroscopy, Crespo-Hernandez et al. observe that the picosecond component of the transient signals recorded for the adenine-thymine oligonucleotide (dA)18.(dT)18 is close to that for (dA)18, but quite different from that for (dAdT)9.(dAdT)9; from this observation, they conclude that excimer formation limits excitation energy to one strand at a time. Here we use time-resolved fluorescence spectroscopy to probe the excited-state dynamics, which reveals the complexity of these systems and indicates that the interpretation of Crespo-Hernandez et al. is an oversimplification. We also comment on the pertinence of separating base stacking and base pairing in excited-state dynamics of double helices and question the authors' assignment of the long-lived signal component found for (dA)18.(dT)18 to adenine excimers.

/pdf/molecular-spectroscopy-complexity-of-excited-state-dynamics-vxh68ndy46.pdf

Molecular spectroscopy: Complexity of excited-state dynamics in DNA

The formation of thymine dimers in the single-stranded oligonucleotide, (dT)20, is studied at room temperature by laser flash photolysis using 266 nm excitation. It is shown that the (6-4) adduct is formed within 4 ms via a reactive intermediate. The formation of cyclobutane dimers is faster than 200 ns. The overall quantum yield for the (6-4) formation is (3.7 ± 0.3) × 10-3, and that of the cyclobutane dimers is (2.8 ± 0.2) × 10-2. No triplet absorption is detected, showing that either the intersystem crossing yield decreases by 1 order of magnitude upon oligomerization (<1.4 × 10-3) or the triplet state reacts with unit efficiency in less than 200 ns to yield cyclobutane dimers.

/pdf/time-resolved-study-of-thymine-dimer-formation-232g1cx0dh.pdf

Time-resolved study of thymine dimer formation.

We report a near-field imaging study of colloidal gold nanocubes. This is accomplished through a photochemical imaging method in which molecular displacements are vectorial in nature, enabling sensitivity to the polarization of the optical near-field of the nanocubes. We analyze the confinement of both electromagnetic hot and “cold” spots with a resolution of λ/35 and emphasize the particularly high spatial confinement of cold spots. The concept of a cold spot complements the well-known electromagnetic hot spot but can have significant advantages. The application of the ultraconfined cold spots to high resolution imaging and spectroscopy is discussed.

Sylvie Marguet

Papers

Fluorescence Properties of DNA Nucleosides and Nucleotides: A Refined Steady-State and Femtosecond Investigation

Influence of disorder on electronic excited states : an experimental and numerical study of alkylthiotriphenylene columnar phases

Molecular spectroscopy: Complexity of excited-state dynamics in DNA

Time-resolved study of thymine dimer formation.

Spatial confinement of electromagnetic hot and cold spots in gold nanocubes.