Conformational effects on vibronic spectra and excited state dynamics of 3-fluorobenzoic acid dimer

Two conformational isomers of 3-fluorobenzoic acid dimer (3-FBA(2)) have been identified in a supersonic jet expansion by use of laser-induced fluorescence excitation (FE), UV-UV hole-burning, and dispersed fluorescence (DF) spectroscopic methods. In the FE spectrum, the S(1) origins of the two isomeric species appear at a frequency gap of only 24 cm(-1), and the vibronic intensities of the redshifted dimer (dimer I) are about two times weaker than those of dimer II. However, ab initio quantum chemistry calculations at the MP2/6-31G(**) level of theory predict that all the isomeric species of 3-FBA(2) have almost the same binding energy (approximately 17 kcal/mol) in the ground state. Furthermore, unlike benzoic acid dimer, the present system shows intense activity for a low-frequency mode in both the FE and DF spectra. With the aid of DFT (B3LYP/6-311G(**)) predicted normal mode frequencies, we have assigned the mode to the in-plane gear (cogwheel) vibration of the cyclic hydrogen-bonded frame of the dimer. The Franck-Condon profiles for vibronic excitation of the mode indicate that the distortion of the cyclic hydrogen bond frame as a result of S(1)<--S(0) excitation is larger for dimer I than dimer II. Moreover, the fluorescence lifetime at the S(1) zero-point level of the former is also significantly smaller than the latter. Using the predictions of configuration interaction singles calculations, we have proposed that the spectral and dynamical differences between the two isomeric species observed in this study are manifestations of the different characteristics of their S(1) surfaces. By measuring FE, DF, and hole-burning spectra of a mixed dimer between 3-fluobenzoic acid and benzoic acid we have shown that the isomeric features in the homodimer spectra are due to two locally excited rotamers of the 3-fluorobenzoic acid moiety.     

Platinum chromophore-based systems for photoinduced charge separation: a molecular design approach for artificial photosynthesis

Photoinduced charge separation is a fundamental step in photochemical energy conversion. In the design of molecularly based systems for light-to-chemical energy conversion, this step is studied through the construction of two- and three-component systems (dyads and triads) having suitable electron donor and acceptor moieties placed at specific positions on a charge-transfer chromophore. The most extensively studied chromophores in this regard are ruthenium(II) tris(diimine) systems with a common 3MLCT excited state, as well as related ruthenium(II) bis(terpyridyl) systems. This Forum contribution focuses on dyads and triads of an alternative chromophore, namely, platinum(II) di- and triimine systems having acetylide ligands. These d8 chromophores all possess a 3MLCT excited state in which the lowest unoccupied molecular orbital is a pi orbital on the heterocyclic aromatic ligand. The excited-state energies of these Pt(II) chromophores are generally higher than those found for the ruthenium(II) tris(diimine) systems, and the directionality of the charge transfer is more certain. The first platinum diimine bis(arylacetylide) triad, constructed by attaching phenothiazene donors to the arylacetylide ligands and a nitrophenyl acceptor to 5-ethynylphenanthroline of the chromophore, exhibited a charge-separated state of 75-ns duration. The first Pt(tpy)(arylacetylide)+-based triad contains a trimethoxybenzamide donor and a pyridinium acceptor and has been structurally characterized. The triad has an edge-to-edge separation between donor and acceptor fragments of 27.95 Angstroms. However, while quenching of the emission is complete for this system, transient absorption (TA) studies reveal that charge transfer does not move onto the pyridinium acceptor. A new set of triads described in detail here and having the formula [Pt(NO2phtpy)(p-C triple-bond C-C6H4CH2(PTZ-R)](PF6), where NO2phtpy = 4'-{4-[2-(4-nitrophenyl)vinyl]phenyl}-2,2';6',2'-terpyridine and PTZ = phenothiazine with R = H, OMe, possess an unsaturated linkage between the chromophore and a nitrophenyl acceptor. While the parent chromophore [Pt(ttpy)(C triple-bond CC6H5)]PF6 is brightly luminescent in a fluid solution at 298 K, the triads exhibit complete quenching of the emission, as do the related donor-chromophore (D-C) dyads. Electrochemically, the triads and D-C dyads exhibit a quasi-reversible oxidation wave corresponding to the PTZ ligand, while the R = H triad and related C-A dyad display a facile quasi-reversible reduction assignable to the acceptor. TA spectroscopy shows that one of the triads possesses a long-lived charge-separated state of approximately 230 ns.     

Synthesis and DNA binding properties of pyrrole amino acid-containing peptides

Dimers of the pyrrole amino acid (Paa), 5-(aminomethyl)pyrrole-2-carboxylic acid, and its derivatives having Lys anchored on N- and C-termini bind in the minor groove of DNA with considerable apparent binding affinities. When the Lys unit is attached to the C-terminus, the resulting ligand binds to ds-DNA with twice the affinity, of the order of 10⁵, than the one carrying two positive charges at the same end.     

Bond Prices Under Information Asymmetry and a Short Rate with Instantaneous Feedback

Methodology and Computing in Applied Probability - We study an information asymmetry problem in a bond market. Especially we derive bond price dynamics of traders with different levels of...