Prediction Of Optical Properties Of Pi-conjugated Organic Materials For Technological Innovations

Organic π-conjugated solids are promising candidates for new optoelectronic materials. The large body of evidence points at their advantageous properties such as high charge-carrier mobility, large nonlinear polarizability, mechanical flexibility, simple and low cost fabrication and superior luminescence. They can be used as nonlinear optical (NLO) materials with large two-photon absorption (2PA) and as electronic components capable of generating nonlinear neutral (excitonic) and charged (polaronic) excitations. In this work, we investigate the appropriate theoretical methods used for the (a) prediction of 2PA properties for rational design of organic materials with improved NLO properties, and (b) understanding of the essential electronic excitations controlling the energy-transfer and charge-transport properties in organic optoelectronics. Accurate prediction of these electro-optical properties is helpful for structureactivity relationships useful for technological innovations. In Chapter 1 we emphasize on the potential use of the organic materials for these two applications. The 2PA process is advantageous over one-photon absorption for deep-tissue fluorescence microscopy, photodynamic therapy, microfabrication and optical data storage owing to the three-dimensional spatial selectivity and improved penetration depth in the absorbing or scattering media. The design of the NLO materials with large 2PA cross-sections may reduce the optical damage due to the use of the high intensity laser beams for excitation. The organic molecules also possess self-localized excited states which can decay radiatively or nonradiatively to form excitonic states. This suggests the use of these materials in the electroluminescent devices such as light-emitting diodes and photovoltaic cells through the processes of exciton formation or dissociation, respectively. It is therefore necessary to understand ultrafast relaxation processes required in understanding the interplay between the iv efficient radiative transfer between the excited states and exciton dissociation into polarons for improving the efficiency of these devices. In Chapter 2, we provide the detailed description of the various theoretical methods applied for the prediction as well as the interpretation of the optical properties of a special class of substituted PPV [poly (p-phenylene vinylene)] oligomers. In Chapter 3, we report the accuracy of different second and third order time dependent density functional theory (TD-DFT) formalisms in prediction of the 2PA spectra compared to the experimental measurements for donor-acceptor PPV derivatives. We recommend a posteriori Tamm-Dancoff approximation method for both qualitative and quantitative analysis of 2PA properties. Whereas, Agren\u27s quadratic response methods lack the double excitations and are not suitable for the qualitative analysis of the state-specific contributions distorting the overall quality of the 2PA predictions. We trace the reasons to the artifactual excited states above the ionization threshold. We also study the effect of the basis set, geometrical constraints and the orbital exchange fraction on the 2PA excitation energies and cross-sections. Higher exchange (BMK and M05-2X) and range-separated (CAM-B3LYP) hybrid functionals are found to yield inaccurate predictions both quantitatively and qualitatively. The failure of the exchangecorrelation (XC) functionals with correct asymptotic is traced to the inaccurate transition dipoles between the valence states, where functionals with low HF exchange succeed. In Chapter 4, we test the performance of different semiempirical wavefunction theory methods for the prediction of 2PA properties compared to the DFT results for the same set of molecules. The spectroscopic parameterized (ZINDO/S) method is relatively better than the general purpose parameterized (PM6) method but the accuracy is trailing behind the DFT methods. The poor performances of PM6 and ZINDO/S methods are attributed to the incorrect description of excited-to-excited state transition and 2PA energies, respectively. The different v semiempirical parameterizations can at best be used for quantitative analysis of the 2PA properties. The ZINDO/S method combined with different orders of multi-reference configuration interactions provide an improved description of 2PA properties. However, the results are observed to be highly dependent on the specific choice for the active space, order of excitation and reference configurations. In Chapter 5, we present a linear response TD-DFT study to benchmark the ability of existing functional models to describe the extent of self-trapped neutral and charged excitations in PPV and its derivative MEH-PPV considered in their trans-isomeric forms. The electronic excitations in question include the lowest singlet (S1) and triplet (T1 † ) excitons, positive (P+ ) and negative (P- ) polarons and the lowest triplet (T1) states. Use of the long-range-corrected DFT functional, such as LC-wPBE, is found to be crucial in order to predict the physically correct spatial localization of all the electronic excitations in agreement with experiment. The inclusion of polarizable dielectric environment play an important role for the charged states. The particlehole symmetry is preserved for both the polymers in trans geometries. These studies indicate two distinct origins leading to self-localization of electronic excitations. Firstly, distortion of molecular geometry may create a spatially localized potential energy well where the state wavefunction self-traps. Secondly, even in the absence of geometric and vibrational dynamics, the excitation may become spatially confined due to energy stabilization caused by polarization effects from surrounding dielectric medium. In Chapter 6, we aim to separate these two fundamental sources of spatial localization. We observe the electronic localization of P + and Pis determined by the polarization effects of the surrounding media and the character of the DFT functional. In contrast, the self-trapping of the electronic wavefunctions of S1 and T1(T1 † ) mostly follows their lattice distortions. Geometry vi relaxation plays an important role in the localization of the S1 and T1 † excitons owing to the nonvariational construction of the excited state wavefunction. While, mean-field calculated P + , Pand T1 states are always spatially localized even in ground state S0 geometry. Polaron P+ and Pformation is signified by the presence of the localized states for the hole or the electron deep inside the HOMO-LUMO gap of the oligomer as a result of the orbital stabilization at the LCwPBE level. The broadening of the HOMO-LUMO band gap for the T1 exciton compared to the charged states is associated with the inverted bond length alternation observed at this level. The molecular orbital energetics are investigated to identify the relationships between state localization and the corresponding orbital structure. In Chapter 7, we investigate the effect of various conformational defects of trans and cis nature on the energetics and localization of the charged P + and Pexcitations in PPV and MEHPPV. We observe that the extent of self-trapping for P+ and Ppolarons is highly sensitive on molecular and structural conformations, and distribution of atomic charges within the polymers. The particle-hole symmetry is broken with the introduction of trans defects and inclusion of the polarizable environment in consistent with experiment. The differences in the behavior of PPV and MEH-PPV is rationalized based on their orbital energetics and atomic charge distributions. We show these isomeric defects influence the behavior and drift mobilities of the charge carriers in substituted PPVs

Moving towards educational leadership

Educational leadership focuses on building a shared vision, improving communication, making decisions collaboratively and remains consistent with the desire to bring about school improvement. Literature reveals that if head teachers / managers’ practices are not consistent with the above-mentioned areas of leadership; the expected outcomes of schools become difficult to achieve. In order to build leadership capacity for school / organizational improvement, Aga Khan University-Institute for Educational Development (AKU-IED) has offered various professional development programmes under the USAID Project III to the collaborating Non-Government Organizations / Community Based Organizations (NGOs / CBOs) working in the education sector of Sindh. These Professional Development Programmes are aimed at facilitating the participants in re-conceptualizing their notions of leadership and encouraging them to foster practices of collegial interaction in setting curriculum, instructions and monitoring evaluation and documentation processes at individual, school and organizational levels. Looking at the observations done by the programme instructional team, during Course Participants (CPs) practicum work, their reflections and presentations, field assignments and also the case studies of few programme graduates, provided us with evidence of some change in educational leadership and management practices. These include moving from individualistic approach to team building exercises, playing active role in curriculum setting, improving monitoring, and evaluation and documentation practices. This presentation highlights the strategies, experiences and challenges of the offered programmes with particular concentration on course participants’ application of the newly acquired knowledge, skills, attitudes and the process of institutionalizing change in their context

Self-trapping of excitons, violation of condon approximation, and efficient fluorescence in conjugated cycloparaphenylenes

Cycloparaphenylenes, the simplest structural unit of armchair carbon nanotubes, have unique optoelectronic properties counterintuitive in the class of conjugated organic materials. Our time-dependent density functional theory study and excited state dynamics simulations of cycloparaphenylene chromophores provide a simple and conceptually appealing physical picture explaining experimentally observed trends in optical properties in this family of molecules. Fully delocalized degenerate second and third excitonic states define linear absorption spectra. Self-trapping of the lowest excitonic state due to electron-phonon coupling leads to the formation of spatially localized excitation in large cycloparaphenylenes within 100 fs. This invalidates the commonly used Condon approximation and breaks optical selection rules, making these materials superior fluorophores. This process does not occur in the small molecules, which remain inefficient emitters. A complex interplay of symmetry, π-conjugation, conformational distortion and bending strain controls all photophysics of cycloparaphenylenes.Fil: Adamska, Lyudmyla. Los Alamos National Laboratory. Los Alamos; Estados UnidosFil: Nayyar, Iffat. Los Alamos National Laboratory. Los Alamos; Estados UnidosFil: Chen, Hang. Boston University; Estados UnidosFil: Swan, Anna K.. Boston University; Estados UnidosFil: Oldani, Andres Nicolas. Universidad Nacional de Quilmes; ArgentinaFil: Fernández Alberti, Sebastián. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes; ArgentinaFil: Golder, Matthew R.. University of Oregon; Estados UnidosFil: Jasti, Ramesh. University of Oregon; Estados UnidosFil: Doorn, Stephen K.. Los Alamos National Laboratory. Los Alamos; Estados UnidosFil: Tretiak, Sergei. Los Alamos National Laboratory. Los Alamos; Estados Unido

Global age-sex-specific mortality, life expectancy, and population estimates in 204 countries and territories and 811 subnational locations, 1950–2021, and the impact of the COVID-19 pandemic: a comprehensive demographic analysis for the Global Burden of Disease Study 2021

Background: Estimates of demographic metrics are crucial to assess levels and trends of population health outcomes. The profound impact of the COVID-19 pandemic on populations worldwide has underscored the need for timely estimates to understand this unprecedented event within the context of long-term population health trends. The Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2021 provides new demographic estimates for 204 countries and territories and 811 additional subnational locations from 1950 to 2021, with a particular emphasis on changes in mortality and life expectancy that occurred during the 2020–21 COVID-19 pandemic period. Methods: 22 223 data sources from vital registration, sample registration, surveys, censuses, and other sources were used to estimate mortality, with a subset of these sources used exclusively to estimate excess mortality due to the COVID-19 pandemic. 2026 data sources were used for population estimation. Additional sources were used to estimate migration; the effects of the HIV epidemic; and demographic discontinuities due to conflicts, famines, natural disasters, and pandemics, which are used as inputs for estimating mortality and population. Spatiotemporal Gaussian process regression (ST-GPR) was used to generate under-5 mortality rates, which synthesised 30 763 location-years of vital registration and sample registration data, 1365 surveys and censuses, and 80 other sources. ST-GPR was also used to estimate adult mortality (between ages 15 and 59 years) based on information from 31 642 location-years of vital registration and sample registration data, 355 surveys and censuses, and 24 other sources. Estimates of child and adult mortality rates were then used to generate life tables with a relational model life table system. For countries with large HIV epidemics, life tables were adjusted using independent estimates of HIV-specific mortality generated via an epidemiological analysis of HIV prevalence surveys, antenatal clinic serosurveillance, and other data sources. Excess mortality due to the COVID-19 pandemic in 2020 and 2021 was determined by subtracting observed all-cause mortality (adjusted for late registration and mortality anomalies) from the mortality expected in the absence of the pandemic. Expected mortality was calculated based on historical trends using an ensemble of models. In location-years where all-cause mortality data were unavailable, we estimated excess mortality rates using a regression model with covariates pertaining to the pandemic. Population size was computed using a Bayesian hierarchical cohort component model. Life expectancy was calculated using age-specific mortality rates and standard demographic methods. Uncertainty intervals (UIs) were calculated for every metric using the 25th and 975th ordered values from a 1000-draw posterior distribution. Findings: Global all-cause mortality followed two distinct patterns over the study period: age-standardised mortality rates declined between 1950 and 2019 (a 62·8% [95% UI 60·5–65·1] decline), and increased during the COVID-19 pandemic period (2020–21; 5·1% [0·9–9·6] increase). In contrast with the overall reverse in mortality trends during the pandemic period, child mortality continued to decline, with 4·66 million (3·98–5·50) global deaths in children younger than 5 years in 2021 compared with 5·21 million (4·50–6·01) in 2019. An estimated 131 million (126–137) people died globally from all causes in 2020 and 2021 combined, of which 15·9 million (14·7–17·2) were due to the COVID-19 pandemic (measured by excess mortality, which includes deaths directly due to SARS-CoV-2 infection and those indirectly due to other social, economic, or behavioural changes associated with the pandemic). Excess mortality rates exceeded 150 deaths per 100 000 population during at least one year of the pandemic in 80 countries and territories, whereas 20 nations had a negative excess mortality rate in 2020 or 2021, indicating that all-cause mortality in these countries was lower during the pandemic than expected based on historical trends. Between 1950 and 2021, global life expectancy at birth increased by 22·7 years (20·8–24·8), from 49·0 years (46·7–51·3) to 71·7 years (70·9–72·5). Global life expectancy at birth declined by 1·6 years (1·0–2·2) between 2019 and 2021, reversing historical trends. An increase in life expectancy was only observed in 32 (15·7%) of 204 countries and territories between 2019 and 2021. The global population reached 7·89 billion (7·67–8·13) people in 2021, by which time 56 of 204 countries and territories had peaked and subsequently populations have declined. The largest proportion of population growth between 2020 and 2021 was in sub-Saharan Africa (39·5% [28·4–52·7]) and south Asia (26·3% [9·0–44·7]). From 2000 to 2021, the ratio of the population aged 65 years and older to the population aged younger than 15 years increased in 188 (92·2%) of 204 nations. Interpretation: Global adult mortality rates markedly increased during the COVID-19 pandemic in 2020 and 2021, reversing past decreasing trends, while child mortality rates continued to decline, albeit more slowly than in earlier years. Although COVID-19 had a substantial impact on many demographic indicators during the first 2 years of the pandemic, overall global health progress over the 72 years evaluated has been profound, with considerable improvements in mortality and life expectancy. Additionally, we observed a deceleration of global population growth since 2017, despite steady or increasing growth in lower-income countries, combined with a continued global shift of population age structures towards older ages. These demographic changes will likely present future challenges to health systems, economies, and societies. The comprehensive demographic estimates reported here will enable researchers, policy makers, health practitioners, and other key stakeholders to better understand and address the profound changes that have occurred in the global health landscape following the first 2 years of the COVID-19 pandemic, and longer-term trends beyond the pandemic

A study of classroom culture and school innovation

The central theme of this study is to understand selected elements of classroom culture and how this might affect the successful implementation of innovations such as Inquiry and Cooperative learning strategies. For the study several classes were observed and interviews were conducted to identify and explore the patterns of classroom culture. Meanings were derived from such elements as: teachers\u27 actions, students\u27 activities, classroom setting, and communications patterns. Available literature helped in deriving these meanings from actions, symbols and norms. To arrive at a conclusion, suggestions made by the interviewees guided me. A new understanding of the classroom was gained and the research provided directions about where to start, what actions and decisions to make before implementing new strategies in the future

Two-Photon Absorption Spectra Predicted By Semiempirical Methods

The molecular markers with large two-photon absorption cross-sections are highly sought for many applications, from photodynamic therapy to optical information processing. One may envision computational design of the chromophores combining desirable one- and two-photon absorption (1PA and 2PA) optical features with other properties important for the specific applications (solubility, toxicity etc.). Semiempirical methods are fast and seem to be well suited for the high throughput computational screening, which may be a part of such rational design strategy. Here we apply several semiempirical methods for prediction of 2PA spectra for substituted olygophenylvinylenes of donor-acceptor type. The predictions are compared to the experiment and previously reported results of the density functional theory (DFT) methods. We find ZINDO/S spectroscopic parameterization combined with single configuration interaction (CIS) method to perform far superior to the general purpose PM6 parameterization combined with various CI schemes. The poor performance in the prediction of 2PA cross-sections at PM6 level is traced to inaccurate excitation energies and incorrect transition dipoles between the 1PA and 2PA states. The state-to-state transition dipoles predicted by the ZINDO/S method are much better, as 2PA cross-sections reach the accuracy comparable to TD-DFT predictions after empirical corrections to the excitation energies. We conclude that semiempirical parameterizations can be used only for qualitative analysis of the 2PA properties. The excitation energies are in need of empirical correction, such as QSPR a linear regression for specific class of molecules. The ZINDO/S parameterization combined with multi-reference CI schemes (such as MR-CIS) could yield a good description of the intensities of 1PA and 2PA excitations, provided the right selection in reference configurations, order of excitation and active space. It can not be considered a black-box method, which makes it unsuitable choice for the high throughput screening technique. Copyright © 2014 American Scientific Publishers All rights reserved

Comparison Of Td-Dft Methods For The Calculation Of Two-Photon Absorption Spectra Of Oligophenylvinylenes

We investigate the accuracy of different formalisms within density functional theory in prediction of two-photon absorption (2PA) spectra for substituted oligophenylvinylenes compared to the experimental measurements. The quadratic response methods are compared with the recently proposed a posteriori Tamm-Dancoff approximation (ATDA) and previously published third-order coupled electronic oscillator results. Quadratic response is found to overestimate the cross sections in all cases. We trace the reasons to unreliable excited state description above the ionization threshold. In addition, quadratic response lacks the double excitations so that their contributions to the 2PA spectra are redistributed over the nearest single character excitations. This distorts the individual contributions to the 2PA response and affects the overall picture. For this reason, we do not recommend quadratic response for the essential state analysis, while ATDA can be used both for the 2PA predictions and the structure/property correlations. As an illustration for ATDA based essential state analysis, we report the mechanism of large 2PA in symmetric donor/acceptor substituted polyphenylvinylene (PPV) oligomers. While HOMO-LUMO transition provides the only bright intermediate state, the brightness of the one-photon absorption (1PA) to 2PA transition is associated with symmetric to asymmetric linear combination of the respective donor (HOMO -1 to HOMO) or acceptor (LUMO to LUMO + 1) fragment orbitals of the donor or acceptor substituents. We also study the effect of the fraction of Hartree-Fock (HF) exchange on 2PA excitation energies and cross sections. Higher exchange (BMK and M05-2X) and range separated (CAM-B3LYP) hybrid functionals are found to yield rather inaccurate predictions both quantitatively and qualitatively. The results obtained with the long-range corrected functional LC-BLYP do not seem to be useful at all. This failure of the exchange-correlation functionals with the correct asymptotic is traced to inaccurate transition dipoles between the valence states, where only functionals with lower HF exchange succeed. A new sum over states (SOS) cutoff procedure is proposed to compensate for the collapse of the higher-lying excited states obtained with the hybrid functionals. © 2013 American Chemical Society

Physi-Sorption of H2 on Pure and Boron–Doped Graphene Monolayers: A Dispersion–Corrected DFT Study

High-surface-area carbons are of interest as potential candidates to store H2 for fuel–cell power applications. Earlier work has been ambiguous and inconclusive on the effect of boron doping on H2 binding energy. Here, we describe a systematic dispersion–corrected density functional theory study to evaluate the effect of boron doping. We observe some enhancement in H2 binding, due to the presence of a defect, such as terminal hydrogen or distortion from planarity, introduced by the inclusion of boron into a graphene ring, which creates hydrogen adsorption sites with slightly increased binding energy. The increase is from −5 kJ/mol H2 for the pure carbon matrix to −7 kJ/mol H2 for the boron–doped system with the boron content of ~7%. The H2 binding sites have little direct interaction with boron. However, the largest enhancement in physi-sorption energy is seen for systems, where H2 is confined between layers at a distance of about 7 Å, where the H2 binding nearly doubles to −11 kJ/mol H2. These findings suggest that interplanar nanoconfinement might be more effective in enhancing H2 binding. Smaller coronene model is shown to be beneficial for understanding the dependence of interaction energy on the structural configurations and preferential H2 binding sites

Comparison of TD-DFT Methods for the Calculation of Two-Photon Absorption Spectra of Oligophenylvinylenes

We investigate the accuracy of different formalisms within density functional theory in prediction of two-photon absorption (2PA) spectra for substituted oligophenylvinylenes compared to the experimental measurements. The quadratic response methods are compared with the recently proposed <i>a posteriori</i> Tamm–Dancoff approximation (ATDA) and previously published third-order coupled electronic oscillator results. Quadratic response is found to overestimate the cross sections in all cases. We trace the reasons to unreliable excited state description above the ionization threshold. In addition, quadratic response lacks the double excitations so that their contributions to the 2PA spectra are redistributed over the nearest single character excitations. This distorts the individual contributions to the 2PA response and affects the overall picture. For this reason, we do not recommend quadratic response for the essential state analysis, while ATDA can be used both for the 2PA predictions and the structure/property correlations. As an illustration for ATDA based essential state analysis, we report the mechanism of large 2PA in symmetric donor/acceptor substituted polyphenylvinylene (PPV) oligomers. While HOMO–LUMO transition provides the only bright intermediate state, the brightness of the one-photon absorption (1PA) to 2PA transition is associated with symmetric to asymmetric linear combination of the respective donor (HOMO – 1 to HOMO) or acceptor (LUMO to LUMO + 1) fragment orbitals of the donor or acceptor substituents. We also study the effect of the fraction of Hartree–Fock (HF) exchange on 2PA excitation energies and cross sections. Higher exchange (BMK and M05-2X) and range separated (CAM-B3LYP) hybrid functionals are found to yield rather inaccurate predictions both quantitatively and qualitatively. The results obtained with the long-range corrected functional LC-BLYP do not seem to be useful at all. This failure of the exchange-correlation functionals with the correct asymptotic is traced to inaccurate transition dipoles between the valence states, where only functionals with lower HF exchange succeed. A new sum over states (SOS) cutoff procedure is proposed to compensate for the collapse of the higher-lying excited states obtained with the hybrid functionals

Effect Of Trans- And Cis-Isomeric Defects On The Localization Of The Charged Excitations In Π-Conjugated Organic Polymers

We use the long-range-corrected hybrid density functional theory models to study the effect of various conformational distortions of weak-trans and strong-cis nature on the spatial localization of charged states in poly(p-phenylene vinylene) (PPV) and its derivative poly[2-methoxy-5-(2′- ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV). The extent of self-trapping of positive (P+) and negative (P-) polarons is observed to be highly sensitive to molecular conformation that, in turn, controls the distribution of atomic charges within the polymers. It is shown that, to reach good agreement with recent experimental data on lattice distortion for P + and P- excitations, the polarization of the medium plays a critical role. The introduction of weak-trans defects along the MEH-PPV chain breaks the observed symmetry for P+ and P- excitations. The P- states exhibit more spatial localization owing to lattice relaxation than their vacuum counterparts in contrast to P+. These observations suggest higher mobilities of holes than that of electrons in MEH-PPV, in agreement with the experimental observations. The predicted binding, reorganization, and solvation energies for PPV and MEH-PPV are analyzed for this difference in the response behavior of holes and electrons for trans and cis distortions. This study allows for a better understanding of charge-transport and photophysical properties in π-conjugated organic materials by analyzing their underlying structure-property correlations. © 2013 Wiley Periodicals, Inc