2,6-Di-<i>tert-</i>butylpyridine Sorption Approach to Quantify the External Acidity in Hierarchical Zeolites

Gó,ra-Marek, Kinga,Tarach, Karolina,Choi, Minkee American Chemical Society 2014 The Journal of Physical Chemistry Part C Vol.118 No.23

<P>This work was aimed to evidence that substituted pyridine, 2,6-di-<I>tert</I>-butylpyridine, is a suitable probe for the quantitative investigation of the external acidity in hierarchically structured zeolites. The 2,6-di-<I>tert</I>-butylpyridine was too large to enter the micropores, even in wide pore zeolites, and nearly no sites in nonmesoporous zeolites were available. Accessibility studies of acid sites in zeolites TNU-9 and BEA involving quantitative IR measurements with hindered 2,6-di-<I>tert</I>-butylpyridine as a probe were performed. The extinction coefficients of the 1615 cm<SUP>–1</SUP> diagnostic bands of 2,6-di-<I>tert</I>-butylpyridine interacting with Brønsted acid sites were determined. Lewis acid sites were not detected with the probe. The accessibility factor (AF) for the 2,6-di-<I>tert</I>-butylpyridine probe molecule was defined as the number of sites detected by adsorption of the dTBPy (external sites) divided by the total amount of acid sites in the studied zeolites as quantified by pyridine sorption. Upon desilication resulting in the fabrication of the secondary mesopores, the enhanced accessibility of the protonic sites was observed. In comparison to the mesoporous zeolites with the secondary system of mesopores generated by alkaline leaching, considerably higher accessibility of protonic sites was evidenced in both ultrathin ZSM-5 and delaminated ITQ-2 zeolite.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2014/jpccck.2014.118.issue-23/jp501928k/production/images/medium/jp-2014-01928k_0008.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp501928k'>ACS Electronic Supporting Info</A></P>

Concentration-Dependent Photoredox Conversion of As(III)/As(V) on Illuminated Titanium Dioxide Electrodes

Monllor-Satoca, Damia&#x301,n,Gó,mez, Roberto,Choi, Wonyong American Chemical Society 2012 Environmental science & technology Vol.46 No.10

<P>The photoconversion of As(III) (arsenite) and As(V) (arsenate) over a mesoporous TiO<SUB>2</SUB> electrode was investigated in a photoelectrochemical (PEC) cell for a wide range of concentrations (μM–mM), under nonbiased (open-circuit potential measurements) and biased (short-circuit current measurements) conditions. Not only As(III) can be oxidized, but also As(V) can be reduced in the anoxic condition under UV irradiation. However, the reversible nature of As(III)/As(V) photoconversion was not observed in the normal air-equilibrated condition because the dissolved O<SUB>2</SUB> is far more efficient as an electron acceptor than As(V). Although As(III) should be oxidized by holes, its presence did not increase the photooxidation current in a monotonous way: the photocurrent was reduced by the presence of As(III) in the micromolar range but enhanced in the millimolar range. This abnormal concentration-dependent behavior is related with the fate of the intermediate As(IV) species which can be either oxidized or reduced depending on the experimental conditions, combined with surface deactivation for the water photooxidation process. The lowering of the photooxidation current in the presence of micromolar As(III) is ascribed to the role of As(IV) as a charge recombination center. Being an electron acceptor, the addition of As(V) consistently lowers the photocurrent in the entire concentration range. A global concentration-dependent mechanism is proposed accounting for all the PEC results and its relation with the photocatalytic oxidation mechanism is discussed.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/esthag/2012/esthag.2012.46.issue-10/es203922g/production/images/medium/es-2011-03922g_0009.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/es203922g'>ACS Electronic Supporting Info</A></P>

Induced Optical Activity of DNA-Templated Cyanine Dye Aggregates: Exciton Coupling Theory and TD-DFT Studies

Maj, Michał,Jeon, Jonggu,Gó,ra, Robert W.,Cho, Minhaeng American Chemical Society 2013 The journal of physical chemistry. A, Molecules, s Vol.117 No.29

<P>Certain cyanine dye molecules have been observed to self-assemble in DNA templates to form large chiral aggregates, which exhibit induced circular dichroism. The structure and circular dichroism (CD) of one such system, aggregates of a cationic DiSC<SUB>2</SUB>(5) cyanine dye, are investigated using the time-dependent Kohn–Sham density functional theory (TD-DFT) and exciton coupling model. A series of TD-DFT calculations on the aggregates with one, two, and four dye molecules clearly shows the onset of CD induced by the helically twisted structure compatible with the minor groove of DNA templates. More simplified exciton coupling model analysis successfully reproduces the major positive Cotton effect observed in the experiment as well as the TD-DFT calculations, but it is unable to capture minor features of the CD spectrum that are closely related to absolute configurations of constituent dyes in the complex. We assess the performance of various methods used for evaluation of the electronic coupling energies between interacting chromophores. Our results confirm that the interchromophore interactions in cyanine dye aggregates are primarily electrostatic in nature and indicate that the exciton coupling model is adequate for studying induced CD of self-assembled aggregates of cyanine dye molecules.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpcafh/2013/jpcafh.2013.117.issue-29/jp309807y/production/images/medium/jp-2012-09807y_0006.gif'></P>

Cobalt Electrolyte/Dye Interactions in Dye-Sensitized Solar Cells: A Combined Computational and Experimental Study

Mosconi, Edoardo,Yum, Jun-Ho,Kessler, Florian,Gó,mez Garci&#x301,a, Carlos J.,Zuccaccia, Cristiano,Cinti, Antonio,Nazeeruddin, Mohammad K.,Gra&#x308,tzel, Michael,De Angelis, Filippo American Chemical Society 2012 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.134 No.47

<P>We report a combined experimental and computational investigation to understand the nature of the interactions between cobalt redox mediators and TiO<SUB>2</SUB> surfaces sensitized by ruthenium and organic dyes, and their impact on the performance of the corresponding dye-sensitized solar cells (DSSCs). We focus on different ruthenium dyes and fully organic dyes, to understand the dramatic loss of efficiency observed for the prototype Ru(II) N719 dye in conjunction with cobalt electrolytes. Both N719- and Z907-based DSSCs showed an increased lifetime in iodine-based electrolyte compared to the cobalt-based redox shuttle, while the organic D21L6 and D25L6 dyes, endowed with long alkoxy chains, show no significant change in the electron lifetime regardless of employed electrolyte and deliver a high photovoltaic efficiency of 6.5% with a cobalt electrolyte. Ab initio molecular dynamics simulations show the formation of a complex between the cobalt electrolyte and the surface-adsorbed ruthenium dye, which brings the [Co(bpy)<SUB>3</SUB>]<SUP>3+</SUP> species into contact with the TiO<SUB>2</SUB> surface. This translates into a high probability of intercepting TiO<SUB>2</SUB>-injected electrons by the oxidized [Co(bpy)<SUB>3</SUB>]<SUP>3+</SUP> species, lying close to the N719-sensitized TiO<SUB>2</SUB> surface. Investigation of the dye regeneration mechanism by the cobalt electrolyte in the Marcus theory framework led to substantially different reorganization energies for the high-spin (HS) and low-spin (LS) reaction pathways. Our calculated reorganization energies for the LS pathways are in excellent agreement with recent data for a series of cobalt complexes, lending support to the proposed regeneration pathway. Finally, we systematically investigate a series of Co(II)/Co(III) complexes to gauge the impact of ligand substitution and of metal coordination (tris-bidentate vs bis-tridentate) on the HS/LS energy difference and reorganization energies. Our results allow us to trace structure/property relations required for further development of cobalt electrolytes for DSSCs.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2012/jacsat.2012.134.issue-47/ja3079016/production/images/medium/ja-2012-079016_0016.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja3079016'>ACS Electronic Supporting Info</A></P>

Distributed Multipolar Expansion Approach to Calculation of Excitation Energy Transfer Couplings

Błasiak, Bartosz,Maj, Michał,Cho, Minhaeng,Gó,ra, Robert W. American Chemical Society 2015 Journal of chemical theory and computation Vol.11 No.7

<P>We propose a new approach for estimating the electrostatic part of the excitation energy transfer (EET) coupling between electronically excited chromophores based on the transition density-derived cumulative atomic multipole moments (TrCAMM). In this approach, the transition potential of a chromophore is expressed in terms of truncated distributed multipolar expansion and analytical formulas for the TrCAMMs are derived. The accuracy and computational feasibility of the proposed approach is tested against the exact Coulombic couplings, and various multipole expansion truncation schemes are analyzed. The results of preliminary calculations show that the TrCAMM approach is capable of reproducing the exact Coulombic EET couplings accurately and efficiently and is superior to other widely used schemes: the transition charges from electrostatic potential (TrESP) and the transition density cube (TDC) method.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jctcce/2015/jctcce.2015.11.issue-7/acs.jctc.5b00216/production/images/medium/ct-2015-00216f_0004.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ct5b00216'>ACS Electronic Supporting Info</A></P>