A two-state model of twisted intramolecular chargetransfer in monomethine dyes

A two-state model Hamiltonian is proposed to model the coupling of twisting displacements to charge-transfer behavior in the ground and excited states of a general monomethine dye molecule. This coupling may be relevant to the molecular mechanism of environment-dependent fluorescence yield enhancement. The model is parameterized against quantum chemical calculations on different protonation states of the green fluorescent protein chromophore (GFP), which are chosen to sample different regimes of detuning from the cyanine (resonant) limit. The model provides a simple yet realistic description of the charge transfer character along two possible excited state twisting channels associated with the methine bridge. It describes qualitatively different behavior in three regions that can be classified by their relationship to the resonant (cyanine) limit. The regimes differ by the presence or absence of twist-dependent polarization reversal and the occurrence of conical intersections. We find that selective biasing of one twisting channel over another by an applied diabatic biasing potential can only be achieved in a finite range of parameters near the cyanine limit.Comment: 45 pages, 9 Figures (incl. 2 chemical schemes). Accepted for publication by the Journal of Chemical Physics. Changes include 2 additional figures to and expanded discussion of key points felt to be important, and condensed discussion of some points felt to be less importan

Bond Alternation, Polarizability and Resonance Detuning in Methine Dyes

We derive structure-property relationships for methine ("Brooker") dyes relating the color of the dye and its symmetric parents to its bond alternation in the ground state and also to the dipole properties associated with its low-lying charge-resonance (or charge-transfer) transition. We calibrate and test these relationships on an array of different protonation states of the green fluorescent protein chromophore motif (an asymmetric halochromic methine dye) and its symmetric parent dyes. The relationships rely on the assumption that the diabatic states that define the Platt model for methine dye color [J.R. Platt, J. Chem. Phys. 25 80 (1956)] can also be distinguished by their single-double bond alternation and by their charge localization character. These assumptions are independent of the primary constraint that defines the diabatic states in the Platt model - specifically, the Brooker deviation rule for methine dyes [L.G.S. Brooker, Rev. Mod. Phys. 14 275 (1942)]. Taking these assumptions, we show that the Platt model offers an alternate route to known structure-property relationships between the bond length alternation and the quadratic nonlinear polarizability {\beta}. We show also that the Platt model can be parameterized without the need for synthesis of the symmetric parents of a given dye, using dipole data obtained through spectroscopic measurements. This suggests that the Platt model parameters may be used as independent variables in free-energy relationships for chromophores whose symmetric parents cannot be synthesized or chromophores strongly bound to biomolecular environments. The latter category includes several recently characterized biomolecular probe constructs. We illustrate these concepts by an analysis of previously reported electroabsorption and second-harmonic generation experiments on green fluorescent proteins.Comment: Keywords: Structure-property relationships, Brooker dyes, methine dyes, push-pull polyenes, nonlinear optical response, bond length alternation, bond order - bond length relationships, fluorescent protein

Valence-bond Non-equilibrium Solvation Model for a Twisting Monomethine Cyanine

We propose and analyze a two-state valence-bond model of non-equilibrium solvation effects on the excited-state twisting reaction of monomethine cyanines. Suppression of this reaction is thought responsible for environment-dependent fluorescence yield enhancement in these dyes. Fluorescence is quenched because twisting is accompanied via the formation of dark twisted intramolecular charge-transfer (TICT) states. For monomethine cyanines, where the ground state is a superposition of structures with different bond and charge localization, there are two possible twisting pathways with different charge localization in the excited state. For parameters corresponding to symmetric monomethines, the model predicts two low-energy twisting channels on the excited-state surface that lead to a manifold of twisted intramolecular charge-transfer (TICT) states. For typical monomethines, twisting on the excited state surface will occur with a small barrier or no barrier. Changes in the solvation configuration can differentially stabilize TICT states in channels corresponding to different bonds, and that the position of a conical intersection between adiabatic states moves in response to solvation to stabilize either one channel or the other. There is a conical intersection seam that grows along the bottom of the excited-state potential with increasing solvent polarity. For monomethine cyanines with modest-sized terminal groups in moderately polar solution, the bottom of the excited-state potential surface is completely spanned by a conical intersection seam.Comment: To appear in the Journal of Chemical Physic

A Diabatic Three-State Representation of Photoisomerization in the Green Fluorescent Protein Chromophore

We give a quantum chemical description of bridge photoisomerization reaction of green fluorescent protein (GFP) chromophores using a representation over three diabatic states. Bridge photoisomerization leads to non-radiative decay, and competes with fluorescence in these systems. In the protein, this pathway is suppressed, leading to fluorescence. Understanding the electronic structure of the photoisomerization is a prerequisite to understanding how the protein suppresses this pathway and preserves the emitting state of the chromophore. We present a solution to the state-averaged complete active space problem, which is spanned at convergence by three fragment-localized orbitals. We generate the diabatic-state representation by applying a block diagonalization transformation to the Hamiltonian calculated for the anionic chromophore model HBDI with multi-reference, multi-state perturbation theory. The diabatic states that emerge are charge-localized structures with a natural valence-bond interpretation. At planar geometries, the diabatic picture recaptures the charge transfer resonance of the anion. The strong S0-S1 excitation at these geometries is reasonably described within a two-state model, but extension to a three-state model is necessary to describe decay via two possible pathways associated with photoisomerization of the (methine) bridge. Parametric Hamiltonians based on the three-state ansatz can be fit directly to data generated using the underlying active space. We provide an illustrative example of such a parametric Hamiltonian

Star Clusters in Virgo and Fornax Dwarf Irregular Galaxies

We present the results of a search for clusters in dwarf irregular galaxies in the Virgo and Fornax Cluster using HST WFPC2 snapshot data. The galaxy sample includes 28 galaxies, 11 of which are confirmed members of the Virgo and Fornax clusters. In the 11 confirmed members, we detect 237 cluster candidates and determine their V magnitudes, V-I colors and core radii. After statistical subtraction of background galaxies and foreground stars, most of the cluster candidates have V-I colors of -0.2 and 1.4, V magnitudes lying between 20 and 25th magnitude and core radii between 0 and 6 pc. Using H-alpha observations, we find that 26% of the blue cluster candidates are most likely HII regions. The rest of the cluster candidates are most likely massive (>10^4 Msol) young and old clusters. A comparison between the red cluster candidates in our sample and the Milky Way globular clusters shows that they have similar luminosity distributions, but that the red cluster candidates typically have larger core radii. Assuming that the red cluster candidates are in fact globular clusters, we derive specific frequencies (S_N) ranging from ~0-9 for the galaxies. Although the values are uncertain, seven of the galaxies appear to have specific frequencies greater than 2. These values are more typical of ellipticals and nucleated dwarf ellipticals than they are of spirals or Local Group dwarf irregulars.Comment: 46 pages, 14 figures, 3 tables, accepted by AJ. Higher quality PS version of entire paper available at http://www.astro.washington.edu/seth/dirr_gcs.htm

Sparse observations induce large biases in estimates of the global ocean CO2 sink: an ocean model subsampling experiment

Estimates of ocean CO2 uptake from global ocean biogeochemistry models and pCO2-based data products differ substantially, especially in high latitudes and in the trend of the CO2 uptake since 2000. Here, we assess the effect of data sparsity on two pCO2-based estimates by subsampling output from a global ocean biogeochemistry model. The estimates of the ocean CO2 uptake are improved from a sampling scheme that mimics present-day sampling to an ideal sampling scheme with 1000 evenly distributed sites. In particular, insufficient sampling has given rise to strong biases in the trend of the ocean carbon sink in the pCO2 products. The overestimation of the CO2 flux trend by 20-35% globally and 50-130% in the Southern Ocean with the present-day sampling is reduced to less than 15% with the ideal sampling scheme. A substantial overestimation of the decadal variability of the Southern Ocean carbon sink occurs in one product and appears related to a skewed data distribution in pCO2 space. With the ideal sampling, the bias in the mean CO2 flux is reduced from 9-12% to 2-9% globally and from 14-26% to 5-17% in the Southern Ocean. On top of that, discrepancies of about 0.4 PgC yr-1 (15%) persist due to uncertainties in the gas-exchange calculation. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'

Differences between surface and column atmospheric CO_2 and implications for carbon cycle research

We used a three‐dimensional atmospheric transport model to investigate several aspects of column CO_2 that are important for the design of new satellite‐based observation systems and for the interpretation of observations collected by Sun‐viewing spectrometers. These aspects included the amplitude of the diurnal cycle and how it is related to surface fluxes, the amplitude and phase of the seasonal cycle, and the magnitude of the north‐south hemispheric gradient. In our simulation, we found that column CO_2 had less variability than surface CO_2 on all scales. The annual mean column CO_2 north‐south gradient and seasonal cycle amplitude were approximately one half of their surface counterparts and the column CO_2 diurnal amplitude rarely exceeded 1 ppm. A 1 Gt C yr^(−1) Northern Hemisphere carbon sink decreased the north‐south column CO_2 gradient by ∼0.4 ppm

The Contribution of TP-AGB and RHeB Stars to the Near-IR Luminosity of Local Galaxies: Implications for Stellar Mass Measurements of High Redshift Galaxies

Using high spatial resolution HST WFC3 and ACS imaging of resolved stellar populations, we constrain the contribution of thermally-pulsing asymptotic giant branch (TP-AGB) stars and red helium burning (RHeB) stars to the 1.6 um near-infrared (NIR) luminosities of 23 nearby galaxies. The TP-AGB phase contributes as much as 17% of the integrated F160W flux, even when the red giant branch is well populated. The RHeB population contribution can match or even exceed the TP-AGB contribution, providing as much as 21% of the integrated F160W light. The NIR mass-to-light (M/L) ratio should therefore be expected to vary significantly due to fluctuations in the star formation rate over timescales from 25 Myr to several Gyr. We compare our observational results to predictions based on optically derived star formation histories and stellar population synthesis (SPS) models, including models based on the Padova isochrones (used in popular SPS programs). The SPS models generally reproduce the expected numbers of TP-AGB stars in the sample. The same SPS models, however, give a larger discrepancy in the F160W flux contribution from the TP-AGB stars, over-predicting the flux by a weighted mean factor of 2.3 +/-0.8. This larger offset is driven by the prediction of modest numbers of high luminosity TP-AGB stars at young (<300 Myrs) ages. The best-fit SPS models simultaneously tend to under-predict the numbers and fluxes of stars on the RHeB sequence, typically by a factor of 2.0+/-0.6 for galaxies with significant numbers of RHeBs. Coincidentally, over-prediction of the TP-AGB and under-prediction of the RHeBs result in a NIR M/L ratio largely unchanged for a rapid star formation rate. However, the NIR-to-optical flux ratio of galaxies could be significantly smaller than AGB-rich models would predict, an outcome that has been observed in some intermediate redshift post-starburst galaxies. (Abridged)Comment: 28 Pages, 12 Figures, 5 Tables, Accepted for Publication in the Astrophysical Journa