No abstract

Conjugated polymers have an enormous range of potential applications, and recent work shows how many of these can be realized practically. Encapsulation can offer advantages in terms of separating polymer chains to inhibit interchain effects, improving the mechanical, thermal, and chemical stabilities of the assemblies, and possibly allowing individual molecular wires to be addressed. The Figure shows a schematic representation of a polyrotaxane, i.e., a polymer threaded into a host tube and held in place by end caps.

We report an atomic resolution X-ray crystal structure containing both enantiomers of rac-[Ru(phen)2dppz](2+) with the d(ATGCAT)2 DNA duplex (phen = phenanthroline; dppz = dipyridophenazine). The first example of any enantiomeric pair crystallized with a DNA duplex shows different orientations of the Λ and Δ binding sites, separated by a clearly defined structured water monolayer. Job plots show that the same species is present in solution. Each enantiomer is bound at a TG/CA step and shows intercalation from the minor groove. One water molecule is directly located on one phenazine N atom in the Δ-enantiomer only.

C(0 buckminsterfullerene exhibits a large ultrafast third-order nonlinear optical response. Degenerate four-wave-mixing measurements in C(0-benzene solutions were performed using 50-psec pulses at l.064Itm. The magnitude of the nonlinear susceptibility per C60 molecule is~y~=1.5&& IO ' m'V ' and of the same size as that observed in polydiacetylene.In contrast to conjugated polymers, however, a dominant positive real part of the nonlinearity is found, 3 times larger than the imaginary component. The nonlinear response can be described within the model of a free electron in a spherical box, confirming the complete delocalization of electrons on the C(,0 molecule. PACS numbers: 42.65. -k, 33.20.Ea, 36.90.+f Confinement of free or quasifree electrons on a nanometer scale has received considerable interest in optics recently [1]. In semiconductor nanoparticles, for example, carrier confinement leads to a singularity in the density of states at the band gap which in turn gives rise to an enhanced nonlinear optical response near the associated optical transition compared with nonconfined materials [2]. Such particles are commonly termed quantum dots. On the other hand, conjugated polymers such as polyacetylene and polydiacetylene show sizable ultrafast nonlinearities due to the one-dimensional nature of the delocalized tr electrons [3]. Fullerenes [4-9] also possess highly delocalized electrons and so are expected to exhibit nonlinear optical behavior similar to conjugated polymers. In addition, however, the three-dimensional nature of the particles should also be apparent in this response. In this Letter we report the observation of infrared nonlinear optical response of C60-buckminsterfullerenebenzene solutions which shows some similarity with that of conjugated polymers. As a result of a dominant real part of the nonlinearity and the relatively small size of the molecule, however, more favorable solid-state properties can be expected, making this class of materials interesting candidates for nonlinear optical devices. C60 buckminsterfullerene was prepared and purified as described in the literature [8,9]. Magenta solutions up to a maximum concentration of 500 mg/L were prepared in benzene. The absorption spectra of the samples are complex consisting of a series of closely overlapping peaks between 450 and 700 nm with a first absorption maximum near 593 nm. In general, the similarity of this spectrum with that of 3-BCMU polydiacetylene in chlorobenzene should be noted [10].Nonlinear optical measurements were performed using the forward degenerate four-wave-mixing technique as described elsewhere [11]. The laser used was a passively mode-locked amplified Nd-doped yttrium-aluminumgarnet laser emitting 50-psec pulses at 1.064 pm of up to 5 mJ energy. This technique is based on diffraction from nonlinear optically induced transient gratings and allows the magnitude of the nonlinear optical response to be measured. The observed response corresponds to a modulation of the material s refractive index associated with the ligh...

To understand the molecular origins of diseases caused by ultraviolet and visible light, and also to develop photodynamic therapy, it is important to resolve the mechanism of photoinduced DNA damage. Damage to DNA bound to a photosensitizer molecule frequently proceeds by one-electron photo-oxidation of guanine, but the precise dynamics of this process are sensitive to the location and the orientation of the photosensitizer, which are very difficult to define in solution. To overcome this, ultrafast time-resolved infrared (TRIR) spectroscopy was performed on photoexcited ruthenium polypyridyl-DNA crystals, the atomic structure of which was determined by X-ray crystallography. By combining the X-ray and TRIR data we are able to define both the geometry of the reaction site and the rates of individual steps in a reversible photoinduced electron-transfer process. This allows us to propose an individual guanine as the reaction site and, intriguingly, reveals that the dynamics in the crystal state are quite similar to those observed in the solvent medium.