Effect of π-electron conjugation length on the solvent-dependent S1 lifetime of peridinin

Peridinin exhibits an anomalous solvent dependence of its S₁ excited state lifetime attributed to the presence of an intramolecular charge transfer (ICT) state. The nature of this state has yet to be elucidated. Ultrafast time-resolved optical spectroscopy has been performed on a synthetic analog, C₃₅-peridinin, having one less conjugated double bond than peridinin. The data reveal the lifetime decreases from 1.5 ns in n-hexane to 9.2 ps in methanol, an order of magnitude larger than peridinin. This is the strongest solvent dependence on the lifetime of an S₁ state of a carotenoid yet reported. The data support the view that the S₁ and ICT states are strongly coupled.     

Cyanide-bridged Os(II)/Ln(III) coordination networks containing [Os(phen)(CN)4]2- as an energy donor: structural and photophysical properties

Slow evaporation of aqueous solutions containing mixtures of Na 2[Os(phen)(CN) 4], Ln(III) salts (Ln = Pr, Nd, Gd, Er, Yb), and (in some cases) an additional ligand such as 1,10-phenanthroline (phen) or 2,2'-bipyrimidine (bpym) afforded crystalline coordination networks in which the [Os(phen)(CN) 4] (2-) anions are coordinated to Ln(III) cations via Os-CN-Ln cyanide bridges. The additional diimine ligands, if present, also coordinate to the Ln(III) centers. Several types of structure have been identified by X-ray crystallographic studies. Photophysical studies showed that the characteristic emission of the [Os(phen)(CN) 4] (2-) chromophore, which occurs at approximately 680 nm in this type of coordination environment with a triplet metal-to-ligand charge transfer ( (3)MLCT) energy content of approximately 16 000 cm (-1), is quenched by energy transfer to those Ln(III) centers (Pr, Nd, Er, Yb) that have low-lying f-f states capable of accepting energy from the Os(II)-based (3)MLCT state. Time-resolved studies on the residual (partially quenched) Os(II)-based luminescence allowed the rates of Os(II) --> Ln(III) energy transfer to be evaluated. The measured rates varied substantially, having values of >5 x 10 (8), approximately 1 x 10 (8), and 2.5 x 10 (7) s (-1) for Ln = Nd, Er or Yb, and Pr, respectively. These differing rates of Os(II) --> Ln(III) energy transfer can be rationalized on the basis of the availability of f-f states of the different Ln(III) centers that are capable of acting as energy acceptors. In general, the rates of Os(II) --> Ln(III) energy transfer are an order of magnitude faster than the rates of Ru(II) --> Ln(III) energy transfer in a previously described series of [Ru(bipy)(CN) 4] (2-)/Ln(III) networks. This is ascribed principally to the lower energy of the Os(II)-based (3)MLCT state, which provides better spectroscopic overlap with the low-lying f-f states of the Ln(III) ions.     

Multiple charge-transfer emissions from different metal-ligand pairs in ruthenium diimines

Ruthenium diimines are unique in their emissivity. Optical excitation with light of less than 500 nm leads to a strong emission in the 600-700 nm range. All emissive ruthenium complexes appear to undergo intersystem crossing from the absorptive singlet metal-to-ligand charge-transfer (MLCT) state to an emissive triplet MLCT state localized on the lowest-energy metal-ligand pair. In contrast to this currently accepted model, in which a single emissive state is populated and then equilibrates among other states based on a particular set of conditions, the excitation-wavelength dependence of the [(bpy)2RudppH]3+ emission suggests two emissive pathways. One populates an emissive MLCT state localized on a bpy-Ru pair, and the other populates a lower-energy MLCT state localized on the dpp-Ru pair.     

Platinum(II) diimine complexes with catecholate ligands bearing imide electron-acceptor groups: synthesis, crystal structures, (spectro)electrochemical and EPR studies, and electronic structure

A series of catechols with attached imide functionality (imide = phthalimide PHT, 1,8-naphthalimide NAP, 1,4,5,8-naphthalenediimide NDI, and NAP-NDI) has been synthesized and coordinated to the Pt (II)(bpy*) moiety, yielding Pt(bpy*)(cat-imide) complexes (bpy* = 4,4'-di- tert-butyl-2,2'-bipyridine). X-ray crystal structures of PHT and NAP complexes show a distorted square-planar arrangement of ligands around the Pt center. Both complexes form "head-to-tail" dimers in the solid state through remarkably short unsupported Pt...Pt contacts of 3.208 (PHT) and 3.378 A (NAP). The Pt(bpy*)(cat-imide) complexes are shown to combine optical (absorption) and electrochemical properties of the catecholate (electron-donor) and imide (electron-acceptor) groups. The complexes show a series of reversible reduction processes in the range from -0.5 to -1.9 V vs Fc (+)/Fc, which are centered on either bpy* or imide groups, and a reversible oxidation process at +0.07 to +0.14 V, which is centered on the catecholate moiety. A combination of UV-vis absorption spectroscopy, cyclic voltammetry, UV-vis spectroelectrochemistry, and EPR spectroscopy has allowed assignment of the nature of frontier orbitals in Pt(bpy*)(cat-imide) complexes. The HOMO in Pt(bpy*)(cat-imide) is centered on the catechol ligand, while the LUMO is localized either on bpy* or on the imide group, depending on the nature of the imide group involved. Despite the variations in the nature of the LUMO, the lowest-detectable electronic transition in all Pt(bpy*)(cat-imide) complexes has predominantly ligand-to-ligand (catechol-to-diimine) charge-transfer nature (LLCT) and involves a bpy*-based unoccupied molecular orbital in all cases. The LLCT transition in all Pt(bpy*)(cat-imide) complexes appears at 530 nm in CH2Cl2 and is strongly negatively solvatochromic. The energy of this transition is remarkably insensitive to the imide group present, indicating lack of electronic communication between the imide and the catechol moieties within the cat-imide ligand. The high extinction coefficient, approximately 6 x 10(3) L mol(-1) cm(-1) of this predominantly LLCT transition is the result of the Pt orbital contribution, as revealed by EPR spectroscopy of the complexes in various redox states. The CV profile of the oxidation process of Pt(bpy*)(cat-imide) in CH2Cl2 and DMF is concentration dependent, as was shown for NDI and PHT complexes as typical examples. Oxidation appears as a simple diffusion-limited process at low concentrations, with an increasing anodic-to-cathodic peak separation eventually resolving as two independent consecutive waves as the concentration of the complex increases. It is suggested that aggregation of the complexes in the diffusion layer in the course of oxidation is responsible for the observed concentration dependence. Overall, the Pt(bpy*)(cat-imide) complexes are electrochromic compounds in which a series of stepwise reversible redox processes in the potential range from 0.2 to -2 V (vs Fc (+)/Fc) leads to tuneable absorbencies between 300 and 850 nm.     

Modifying the OPLS-AA force field to improve hydration free energies for several amino acid side chains using new atomic charges and an off-plane charge model for aromatic residues

The hydration free energies of amino acid side chains are an important determinant of processes that involve partitioning between different environments, including protein folding, protein complex formation, and protein-membrane interactions. Several recent papers have shown that calculated hydration free energies for polar and aromatic residues (Trp, His, Tyr, Asn, Gln, Asp, Glu) in several common molecular dynamics force fields differ significantly from experimentally measured values. We have attempted to improve the hydration energies for these residues by modifying the partial charges of the OPLS-AA force field based on natural population analysis of density functional theory calculations. The resulting differences between calculated hydration free energies and experimental results for the seven side chain analogs are less than 0.1 kcal/mol. Simulations of the synthetic Trp-rich peptide Trpzip2 show that the new charges lead to significantly improved geometries for interacting Trp-side chains. We also investigated an off-plane charge model for aromatic rings that more closely mimics their electronic configuration. This model results in an improved free energy of hydration for Trp and a somewhat altered benzene-sodium potential of mean force with a more favorable energy for direct benzene-sodium contact.     

Plasma surface functionalization of poly[bis(2,2,2-trifluoroethoxy)phosphazene] films and nanofibers

Polyphosphazenes are a class of hybrid organic-inorganic macromolecules with high thermo-oxidative stability and good solubility in many solvents. Fluoroalkoxy phosphazene polymers also have high surface hydrophobicity. A method is described to tune this surface property while maintaining the advantageous bulk materials characteristics. The polyphosphazene single-substituent polymer, poly[bis(2,2,2-trifluoroethoxy)phosphazene], with flat film, fiber mat, or bead mat morphology was surface functionalized using an atmospheric plasma treatment with oxygen, nitrogen, methane, or tetrafluoromethane/hydrogen gases. Surface chemistry changes were detected by static water contact angle (WCA) measurements as well as X-ray photon spectroscopy (XPS). It was found that changes in the WCA of as much as 150 degrees occurred, accompanied by shifts in the ratio of elements on the polymer surface as detected by XPS. Overall this plasma technique provides a convenient method for the generation of specific surface characteristics while maintaining the hydrophobicity of the bulk material.     

Control of electron transfer in a conjugated porphyrin dimer by selective excitation of planar and perpendicular conformers

A donor-acceptor system is presented in which the electron-transfer rates can be sensitively controlled by means of excitation wavelength and temperature. The electron donor is a butadiyne-linked zinc porphyrin dimer that is connected to a C(60) electron acceptor. The broad distribution of conformations allowed by the butadiyne linker makes it possible to selectively excite perpendicular or planar donor conformers and thereby prepare separate initial states with driving forces for electron transfer that differ by almost 0.2 eV. This, as well as significant differences in electronic coupling, leads to distinctly different rate constants for electron transfer, which in consequence can be controlled by changing excitation wavelength. By extending the system with a secondary donor (ferrocene), a second, long-range charge-separated state can be formed. This system has been used to test the influence of conformational heterogeneity on electron transfer mediated by the porphyrin dimer in the ground state. It was found that if the dimer is forced to a planar conformation by means of a bidentate ligand, the charge recombination rate increased by an order of magnitude relative to the unconstrained system. This illustrates how control of conformation of a molecular wire can affect its behaviour.     

Photophysical and structural properties of cyanoruthenate complexes of hexaazatriphenylene

The tritopic bridging ligand hexaazatriphenylene (HAT) has been used to prepare the mono-, di-, and trinuclear cyanoruthenate complexes [Ru(CN)(4)(HAT)](2-) ([1](2-)), [{Ru(CN)(4)}(2)(mu(2)-HAT)](4-) ([2](4-)), and [{Ru(CN)(4)}(3)(mu(3)-HAT)](6-) ([3](6-)). These complexes are of interest both for their photophysical properties and ability to act as sensitizers, associated with strong MLCT absorptions; and their structural properties, with up to 12 externally directed cyanide ligands at a single "node" for preparation of coordination networks. The complexes are strongly solvatochromic, with broad and intense MLCT absorption manifolds arising from the presence of low-lying pi* orbitals on the HAT ligand, as confirmed by DFT calculations; in aprotic solvents [3](6-) is a panchromatic absorber of visible light. Although nonluminescent in fluid solution, the lowest MLCT excited states have lifetimes in D(2)O of tens of nanoseconds and could be detected by time-resolved IR spectrosocopy. For dinuclear [2](4-) and trinuclear [3](6-) the TRIR spectra are indicative of asymmetric MLCT excited states containing distinct Ru(III) and Ru(II) centers on the IR time scale. The complexes show red (3)MLCT luminescence as solids and in EtOH/MeOH glass at 77 K. Ln(III) salts of [1](2-), [2](4-), and [3](6-) form infinite coordination networks based on Ru-CN-Ln bridges with a range of one-, two-, and three-dimensional polymeric structures. In the Yb(III) and Nd(III) salts of [3](6- )the complex anion forms an 8-connected node. Whereas all of the Gd(III) salts show strong (3)MLCT luminescence in the solid state, the Ru-based emission in the Nd(III) and Yb(III) analogues is substantially quenched by Ru --> Ln photoinduced energy transfer, which results in sensitized near-infrared luminescence from Yb(III) and Nd(III).     

Isolation and structural characterization of a family of endohedral fullerenes including the large, chiral cage fullerenes Tb(3)N@C(88) and Tb(3)N@C(86) as well as the I(h) and D(5)(h) isomers of Tb(3)N@C(80)

The recent finding that isomer 2 of Tb(3)N@C(84) uses one of the 51,568 possible nonisolated pentagon rule (non-IPR) structures for the C(84) cage rather than one of the 24 cage isomers that do obey the IPR suggests that further experimental work on the structure of larger endohedrals is needed to observe the utility of the IPR rule in this uncharted territory. The structures of the newly synthesized endohedral fullerenes--Tb(3)N@C(88), Tb(3)N@C(86), and the Ih and D(5)(h) isomers of Tb(3)N@C(80)--have been determined by single-crystal X-ray diffraction on samples cocrystallized with NiII(octaethylporphyrin). In contrast to the situation for isomer 2 of Tb(3)N@C(84), the structures of Tb(3)N@C(88) and Tb(3)N@C(86) do conform to the IPR. Both Tb(3)N@C(88) and Tb(3)N@C(86) have chiral structures with D(2) symmetry for Tb(3)N@C(880 and D(3) symmetry for Tb(3)N@C(86). Within this group of endohedrals, the size of the carbon cage affects the Tb-N and Tb-C distances, the orientations of the carbon cage with respect to the porphyrin plane, the locations of the metal ions and their orientations relative to the porphyrin plane, and the degree of pyramidalization of the Tb(3)N unit.