1,720,981 research outputs found
Accurate Analysis of Spectrophotometric Data to Quantify the Photoswitching Performance of a Biomimetic E/Z Light-Induced Molecular Switch
No full textPhotoswitches are widely investigated molecules because upon exposure to selected light irradiation, they are able to undergo structural, and hence optical, changes. To fully exploit their responsiveness to irradiation, the quantum efficiency of the forward and back reactions is a fundamental parameter, whose accurate determination is critical. In this work, the spectral evolution of a biomimetic switch, which undergoes E/Z photoinduced isomerization, is spectrophotometrically examined. The minimal spectral differences between the E and Z forms inhibit the accurate determination of the quantum efficiency of the E-Z and Z-E photoconversions. We present a kinetic analysis of the E-Z spectral evolution, which enables us to determine quantum yields of the forward and back reactions; the data show that the back reaction has a higher efficiency (0.60) than the forward process (0.49), evidencing that irradiation conditions have to be carefully selected to accumulate the Z isomer. The method has been further used to determine the E-Z and Z-E quantum yield in the deprotonated form of the biomimetic switch; the obtained values demonstrate that under basic conditions, the back reaction is relatively more efficient than the forward one even for the deprotonated species. The detailed analysis enables us to establish the conditions to monitor the fluorescence of the E and Z forms, setting the basis for a more sensitive detection of the photoswitch and its transformation
Boosting the Efficiency of Red Thermally Activated Delayed Fluorescence via Conjugation Enhancement in Push-Pull Naphthalimide Derivatives
No full textIn this work, we synthesize a series of push-pull compounds bearing naphthalimide as the electron acceptor and tetraphenylethylene (TPE)/triphenylamine (TPA)/phenothiazine (PTZ) as the electron rich/electron donor units. These moieties are arranged in highly conjugated quadrupolar structures. The structure-property relationships are investigated through a joint experimental time-resolved spectroscopic and computational TD-DFT study. The femtosecond transient absorption and fluorescence up-conversion experiments reveal ultrafast photoinduced intramolecular charge transfer. This is likely the key factor leading to efficient spin-orbit CT-induced intersystem crossing for the TPA- and PTZ-derivatives as well as to small singlet-to-triplet energy gap. Consequently, evidence for a delayed fluorescence component is found together with the main prompt emission in the fluorescence kinetics both in solution and in thin film. The weight of the Thermally Activated Delayed Fluorescence (TADF) is greatly enhanced when these fluorophores are used as guests in solid-state host matrices. TADF is interestingly revealed in the orange-red region of the visible. Such long wavelength emission is here observed with surprisingly large fluorescence quantum yields, thanks to the conjugation enhancement achieved in these newly synthesized structures relative to previous studies. Our findings may be thus promising for the future development of efficient third generation TADF-based OLEDs
Effect of the size of polycyclic aryl groups on the competition between adiabatic/diabatic photoisomerization mechanisms of: Cis -styrylarenes
No full textThe occurrence of adiabatic photoisomerization in the singlet manifold directly from 1Z∗ to 1E∗ has been found to be more common than expected. This mechanism has been experimentally evidenced through a detailed fluorimetric study for a large series of styrylarenes. Its weight on the overall cis-trans photoisomerization has been determined and found to increase when increasing the size of the polycyclic chromophore
Tuning the Fluorescence and the Intramolecular Charge Transfer of Phenothiazine Dipolar and Quadrupolar Derivatives by Oxygen Functionalization
No full textA series of new naphthalimide and phenothiazine-based push-pull systems (NPI-PTZ1-5), in which we structurally modulate the oxidation state of the sulfur atom in the thiazine ring, i.e., S(II), S(IV), and S(VI), was designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The effect of the sulfur oxidation state on the spectral, photophysical, and electrochemical properties was investigated. The steady-state absorption and emission results show that oxygen functionalization greatly improves the optical (absorption coefficient and fluorescence efficiency) and nonlinear optical (hyperpolarizability) features. The cyclic voltammetry experiments and the quantum mechanical calculations suggest that phenothiazine is a stronger electron donor unit relative to phenothiazine-5-oxide and phenothiazine-5,5-dioxide, while the naphthalimide is a strong electron acceptor in all cases. The advanced ultrafast spectroscopic measurements, transient absorption, and broadband fluorescence up conversion give insight into the mechanism of photoinduced intramolecular charge transfer. A planar intramolecular charge transfer (PICT) and highly fluorescent excited state are populated for the oxygen-functionalized molecules NPI-PTZ2,3 and NPI-PTZ5; on the other hand, a twisted intramolecular charge transfer (TICT) state is produced upon photoexcitation of the oxygen-free derivatives NPI-PTZ1 and NPI-PTZ4, with the fluorescence being thus significantly quenched. These results prove oxygen functionalization as a new effective synthetic strategy to tailor the photophysics of phenothiazine-based organic materials for different optoelectronic applications. While oxygen-functionalized compounds are highly fluorescent and promising active materials for current-to-light conversion in organic light-emitting diode devices, oxygen-free systems show very efficient photoinduced ICT and may be employed for light-to-current conversion in organic photovoltaics
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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