Photophysical processes and molecular ordering in organic materials for third generation photovoltaics studied by EPR spectroscopy

The world energy consumption is increasing at an average rate of 2.1 % per year, spurred by the economic growth in most of Asian countries, Europe and Canada. The consequent depletion of fossil fuels reservoirs and the reinforced need of environmental sustainability are making the challenge of clean and renewable energy sources one of the most urgent challenges for humankind. Solar power is among the best candidates for the leading role in the energy revolution, being clean, infinite and well distributed over the planet. For this reason, photovoltaic technologies for electricity production are gaining increasing popularity. Although inorganic silicon solar cells dominate the market of photovoltaics, organic and hybrid materials attract considerable interest since their properties like flexibility, light-weight, transparency and low-cost could make the difference in the raising of solar electricity. So far, these materials could not yet outperform conventional silicon, stimulating intensive scientific research on both the development of new materials and the understanding of the photophysical mechanisms governing the photovoltaic behavior of organic/hybrid semiconductors. In this thesis, a series of new organic and hybrid photoactive materials is studied using Electron Paramagnetic Resonance spectroscopy (EPR). This technique, combined with photoexcitation, allows to unambiguously characterize the photoinduced processes involving the formation of paramagnetic states like radicals and triplet states. As shown in the thesis, EPR can also give useful information about molecular ordering in the materials, which is known to be intimately connected with charge transport properties. Conjugated polymers are known for their semiconducting properties and their blends with strong electron accepting fullerene derivatives are among the best performing organic photovoltaic systems. Donor-acceptor alternating copolymers have been introduced to enhance the light-harvesting properties of the blends. Compared to homopolymers, they usually display a lower crystallinity of the deposited films. Thus, XRD techniques are often not suitable to investigate their molecular ordering features. We apply EPR to the analysis of molecular orientational order in the films of two polymers representative of this class, showing that a consistent degree of preferential orientation occurs with two common deposition methods. Fullerene-free materials for polymer solar cells have been recently introduced and overcome some of the drawbacks of fullerene acceptors like the limited absorption and the poor bandgap tunability. In this framework, we study two blends of electron-donor and acceptor polymers to probe their properties with respect to the common fullerene/donor combination, showing that they avoid charge recombination to triplet states which is an active loss mechanism in fullerene-containing blends. Furthermore, the all-polymer films provide a high degree of orientational order and efficient interaction between the donor and acceptor phases that make them promising alternatives to polymer-fullerene blends. A reduced graphene oxide-triphenylamine covalently-linked nanohybrid is studied as potential photosensitizer for TiO2 in dye-sensitized solar cells, able to improve the conductivity and the stability of the system. EPR shows that efficient photoinduced electron transfer from the sensitizer to the semiconductor occurs, paving the way to this new class of photosensitizers. Finally, we investigate the photoactivity of a supramolecular soft-material, forming a gel, composed of small self-assembling donor and acceptor molecules. In this case, EPR allows to verify the efficiency of charge transport across the supramolecular structures, suggesting appealing semiconducting properties of the material. The results of this thesis show the relevance of EPR for unraveling functional and morphological properties of photovoltaic materials and provide a useful characterization of the photophysics of new systems that may be further explored to bring substantial progresses to the field of organic photovoltaics

Flooding Responses on Grapevine: A Physiological, Transcriptional, and Metabolic Perspective

Studies on model plants have shown that temporary soil flooding exposes roots to a significant hypoxic stress resulting in metabolic re-programming, accumulation of toxic metabolites and hormonal imbalance. To date, physiological and transcriptional responses to flooding in grapevine are poorly characterized. To fill this gap, we aimed to gain insights into the transcriptional and metabolic changes induced by flooding on grapevine roots (K5BB rootstocks), on which cv Sauvignon blanc (Vitis vinifera L.) plants were grafted. A preliminary experiment under hydroponic conditions enabled the identification of transiently and steadily regulated hypoxia-responsive marker genes and drafting a model for response to oxygen deprivation in grapevine roots. Afterward, over two consecutive vegetative seasons, flooding was imposed to potted vines during the late dormancy period, to mimick the most frequent waterlogging events occurring in the field. Untargeted transcriptomic and metabolic profiling approaches were applied to investigate early responses of grapevine roots during exposure to hypoxia and subsequent recovery after stress removal. The initial hypoxic response was marked by a significant increase of the hypoxia-inducible metabolites ethanol, GABA, succinic acid and alanine which remained high also 1 week after recovery from flooding with the exception of ethanol that leveled off. Transcriptomic data supported the metabolic changes by indicating a substantial rearrangement of primary metabolic pathways through enhancement of the glycolytic and fermentative enzymes and of a subset of enzymes involved in the TCA cycle. GO and KEGG pathway analyses of differentially expressed genes showed a general down-regulation of brassinosteroid, auxin and gibberellin biosynthesis in waterlogged plants, suggesting a general inhibition of root growth and lateral expansion. During recovery, transcriptional activation of gibberellin biosynthetic genes and down-regulation of the metabolic ones may support a role for gibberellins in signaling grapevine rootstocks waterlogging metabolic and hormonal changes to the above ground plant. The significant internode elongation measured upon budbreak during recovery in plants that had experienced flooding supported this hypothesis. Overall integration of these data enabled us to draft a first comprehensive view of the molecular and metabolic pathways involved in grapevine\u2019s root responses highlighting a deep metabolic and transcriptomic reprogramming during and after exposure to waterlogging

Flooding Responses on Grapevine: A Physiological, Transcriptional, and Metabolic Perspective

Studies on model plants have shown that temporary soil flooding exposes roots to a significant hypoxic stress resulting in metabolic re-programming, accumulation of toxic metabolites and hormonal imbalance. To date, physiological and transcriptional responses to flooding in grapevine are poorly characterized. To fill this gap, we aimed to gain insights into the transcriptional and metabolic changes induced by flooding on grapevine roots (K5BB rootstocks), on which cv Sauvignon blanc (Vitis vinifera L.) plants were grafted. A preliminary experiment under hydroponic conditions enabled the identification of transiently and steadily regulated hypoxia-responsive marker genes and drafting a model for response to oxygen deprivation in grapevine roots. Afterward, over two consecutive vegetative seasons, flooding was imposed to potted vines during the late dormancy period, to mimick the most frequent waterlogging events occurring in the field. Untargeted transcriptomic and metabolic profiling approaches were applied to investigate early responses of grapevine roots during exposure to hypoxia and subsequent recovery after stress removal. The initial hypoxic response was marked by a significant increase of the hypoxia-inducible metabolites ethanol, GABA, succinic acid and alanine which remained high also 1 week after recovery from flooding with the exception of ethanol that leveled off. Transcriptomic data supported the metabolic changes by indicating a substantial rearrangement of primary metabolic pathways through enhancement of the glycolytic and fermentative enzymes and of a subset of enzymes involved in the TCA cycle. GO and KEGG pathway analyses of differentially expressed genes showed a general down-regulation of brassinosteroid, auxin and gibberellin biosynthesis in waterlogged plants, suggesting a general inhibition of root growth and lateral expansion. During recovery, transcriptional activation of gibberellin biosynthetic genes and down-regulation of the metabolic ones may support a role for gibberellins in signaling grapevine rootstocks waterlogging metabolic and hormonal changes to the above ground plant. The significant internode elongation measured upon budbreak during recovery in plants that had experienced flooding supported this hypothesis. Overall integration of these data enabled us to draft a first comprehensive view of the molecular and metabolic pathways involved in grapevine’s root responses highlighting a deep metabolic and transcriptomic reprogramming during and after exposure to waterlogging

Prevalence, associated factors and outcomes of pressure injuries in adult intensive care unit patients: the DecubICUs study

Funder: European Society of Intensive Care Medicine; doi: http://dx.doi.org/10.13039/501100013347Funder: Flemish Society for Critical Care NursesAbstract: Purpose: Intensive care unit (ICU) patients are particularly susceptible to developing pressure injuries. Epidemiologic data is however unavailable. We aimed to provide an international picture of the extent of pressure injuries and factors associated with ICU-acquired pressure injuries in adult ICU patients. Methods: International 1-day point-prevalence study; follow-up for outcome assessment until hospital discharge (maximum 12 weeks). Factors associated with ICU-acquired pressure injury and hospital mortality were assessed by generalised linear mixed-effects regression analysis. Results: Data from 13,254 patients in 1117 ICUs (90 countries) revealed 6747 pressure injuries; 3997 (59.2%) were ICU-acquired. Overall prevalence was 26.6% (95% confidence interval [CI] 25.9–27.3). ICU-acquired prevalence was 16.2% (95% CI 15.6–16.8). Sacrum (37%) and heels (19.5%) were most affected. Factors independently associated with ICU-acquired pressure injuries were older age, male sex, being underweight, emergency surgery, higher Simplified Acute Physiology Score II, Braden score 3 days, comorbidities (chronic obstructive pulmonary disease, immunodeficiency), organ support (renal replacement, mechanical ventilation on ICU admission), and being in a low or lower-middle income-economy. Gradually increasing associations with mortality were identified for increasing severity of pressure injury: stage I (odds ratio [OR] 1.5; 95% CI 1.2–1.8), stage II (OR 1.6; 95% CI 1.4–1.9), and stage III or worse (OR 2.8; 95% CI 2.3–3.3). Conclusion: Pressure injuries are common in adult ICU patients. ICU-acquired pressure injuries are associated with mainly intrinsic factors and mortality. Optimal care standards, increased awareness, appropriate resource allocation, and further research into optimal prevention are pivotal to tackle this important patient safety threat

Correction to: Prevalence, associated factors and outcomes of pressure injuries in adult intensive care unit patients: the DecubICUs study

The original version of this article unfortunately contained a mistake

Prevalence, associated factors and outcomes of pressure injuries in adult intensive care unit patients: the DecubICUs study

Funder: European Society of Intensive Care Medicine; doi: http://dx.doi.org/10.13039/501100013347Funder: Flemish Society for Critical Care NursesAbstract: Purpose: Intensive care unit (ICU) patients are particularly susceptible to developing pressure injuries. Epidemiologic data is however unavailable. We aimed to provide an international picture of the extent of pressure injuries and factors associated with ICU-acquired pressure injuries in adult ICU patients. Methods: International 1-day point-prevalence study; follow-up for outcome assessment until hospital discharge (maximum 12 weeks). Factors associated with ICU-acquired pressure injury and hospital mortality were assessed by generalised linear mixed-effects regression analysis. Results: Data from 13,254 patients in 1117 ICUs (90 countries) revealed 6747 pressure injuries; 3997 (59.2%) were ICU-acquired. Overall prevalence was 26.6% (95% confidence interval [CI] 25.9–27.3). ICU-acquired prevalence was 16.2% (95% CI 15.6–16.8). Sacrum (37%) and heels (19.5%) were most affected. Factors independently associated with ICU-acquired pressure injuries were older age, male sex, being underweight, emergency surgery, higher Simplified Acute Physiology Score II, Braden score 3 days, comorbidities (chronic obstructive pulmonary disease, immunodeficiency), organ support (renal replacement, mechanical ventilation on ICU admission), and being in a low or lower-middle income-economy. Gradually increasing associations with mortality were identified for increasing severity of pressure injury: stage I (odds ratio [OR] 1.5; 95% CI 1.2–1.8), stage II (OR 1.6; 95% CI 1.4–1.9), and stage III or worse (OR 2.8; 95% CI 2.3–3.3). Conclusion: Pressure injuries are common in adult ICU patients. ICU-acquired pressure injuries are associated with mainly intrinsic factors and mortality. Optimal care standards, increased awareness, appropriate resource allocation, and further research into optimal prevention are pivotal to tackle this important patient safety threat

Photophysical processes and molecular ordering in organic materials for third generation photovoltaics studied by EPR spectroscopy

The world energy consumption is increasing at an average rate of 2.1 % per year, spurred by the economic growth in most of Asian countries, Europe and Canada. The consequent depletion of fossil fuels reservoirs and the reinforced need of environmental sustainability are making the challenge of clean and renewable energy sources one of the most urgent challenges for humankind. Solar power is among the best candidates for the leading role in the energy revolution, being clean, infinite and well distributed over the planet. For this reason, photovoltaic technologies for electricity production are gaining increasing popularity. Although inorganic silicon solar cells dominate the market of photovoltaics, organic and hybrid materials attract considerable interest since their properties like flexibility, light-weight, transparency and low-cost could make the difference in the raising of solar electricity. So far, these materials could not yet outperform conventional silicon, stimulating intensive scientific research on both the development of new materials and the understanding of the photophysical mechanisms governing the photovoltaic behavior of organic/hybrid semiconductors. In this thesis, a series of new organic and hybrid photoactive materials is studied using Electron Paramagnetic Resonance spectroscopy (EPR). This technique, combined with photoexcitation, allows to unambiguously characterize the photoinduced processes involving the formation of paramagnetic states like radicals and triplet states. As shown in the thesis, EPR can also give useful information about molecular ordering in the materials, which is known to be intimately connected with charge transport properties. Conjugated polymers are known for their semiconducting properties and their blends with strong electron accepting fullerene derivatives are among the best performing organic photovoltaic systems. Donor-acceptor alternating copolymers have been introduced to enhance the light-harvesting properties of the blends. Compared to homopolymers, they usually display a lower crystallinity of the deposited films. Thus, XRD techniques are often not suitable to investigate their molecular ordering features. We apply EPR to the analysis of molecular orientational order in the films of two polymers representative of this class, showing that a consistent degree of preferential orientation occurs with two common deposition methods. Fullerene-free materials for polymer solar cells have been recently introduced and overcome some of the drawbacks of fullerene acceptors like the limited absorption and the poor bandgap tunability. In this framework, we study two blends of electron-donor and acceptor polymers to probe their properties with respect to the common fullerene/donor combination, showing that they avoid charge recombination to triplet states which is an active loss mechanism in fullerene-containing blends. Furthermore, the all-polymer films provide a high degree of orientational order and efficient interaction between the donor and acceptor phases that make them promising alternatives to polymer-fullerene blends. A reduced graphene oxide-triphenylamine covalently-linked nanohybrid is studied as potential photosensitizer for TiO2 in dye-sensitized solar cells, able to improve the conductivity and the stability of the system. EPR shows that efficient photoinduced electron transfer from the sensitizer to the semiconductor occurs, paving the way to this new class of photosensitizers. Finally, we investigate the photoactivity of a supramolecular soft-material, forming a gel, composed of small self-assembling donor and acceptor molecules. In this case, EPR allows to verify the efficiency of charge transport across the supramolecular structures, suggesting appealing semiconducting properties of the material. The results of this thesis show the relevance of EPR for unraveling functional and morphological properties of photovoltaic materials and provide a useful characterization of the photophysics of new systems that may be further explored to bring substantial progresses to the field of organic photovoltaics.Il consumo mondiale di energia ha un tasso medio di crescita del 2.1 % all’anno, trainato dalla crescita economica di molti Paesi asiatici, dell’Europa e del Canada. Il conseguente depauperamento delle risorse di combustibili fossili e il più stringente bisogno di proteggere l’ambiente stanno facendo della sfida delle energie rinnovabili una delle più urgenti sfide che l’umanità deve affrontare. L’energia solare è tra i migliori candidati a svolgere il ruolo di punta nella rivoluzione energetica, essendo una fonte di energia pulita, infinita e ben distribuita nel pianeta. Per questo motivo le tecnologie fotovoltaiche per la produzione di energia elettrica stanno acquistando crescente popolarità. Sebbene le celle solari a base di Silicio dominino il mercato del fotovoltaico, materiali organici e ibridi sono fonte di crescente interesse grazie alle loro peculiari proprietà, come la flessibilità, la leggerezza e la trasparenza, il basso costo, che ci si aspetta possano fare la differenza nell’affermazione del fotovoltaico. Fino ad ora questi materiali non hanno superato il rendimento dei materiali convenzionali a base di Silicio, stimolando la ricerca scientifica verso lo sviluppo di nuovi materiali e lo studio dei meccanismi fotofisici che governano il comportamento fotovoltaico dei semiconduttori organici e ibridi. In questa tesi, una serie di nuovi materiali fotoattivi, organici e ibridi, è stata studiata utilizzando la spettroscopia di Risonanza Paramagnetica Elettronica (EPR). Tale tecnica, combinata con la fotoeccitazione, permette di caratterizzare i processi fotoindotti che portano alla formazione di stati paramagnetici come radicali e stati di tripletto. Come mostrato nella tesi, la tecnica EPR può essere anche utilizzata per ottenere informazioni circa l’ordine molecolare nei materiali, che è noto essere strettamente collegato alle loro proprietà di trasporto di carica. I polimeri coniugati sono noti per le loro proprietà di semiconduttori e le loro miscele con derivati fullereneci - forti electron-accettori - sono tra i sistemi fotovoltaici organici più efficienti. Copolimeri alternanti composti da unità elettron-accettrici e donatrici sono stati introdotti per aumentare l’efficienza di assorbimento dello spettro solare. Rispetto ai classici omopolimeri, questi mostrano solitamente una minore cristallinità dei film depositati. Pertanto, tecniche diffrattometriche si rivelano spesso inadeguate per caratterizzarne l’ordine molecolare. In questa tesi l’EPR viene utilizzato per analizzare l’ordine orientazionale in due polimeri rappresentativi di questa classe, mostrando che un grado consistente di orientazione preferenziale è presente nei film ottenuti con due diverse tecniche di deposizione. Materiali fullerene-free per le celle solari polimeriche sono stati recentemente introdotti per superare alcuni degli svantaggi degli accettori fullerenici, come il limitato assorbimento della luce solare e la difficoltà nel regolare il bandgap e le proprietà elettroniche. In questo conteso, abbiamo studiato due blend costituiti da polimeri elettron-accettori e donatori al fine di investigarne le proprietà e di compararle a quelle dei convenzionali blend di polimeri donatori con derivati fullerenici, dimostrando che essi eliminano la ricombinazione di cariche a formare stati di tripletto, meccanismo noto come fonte di perdita di efficienza nei materiali contenti fullereni. Inoltre, i film polimerici mostrano un elevato grado di ordine orientazionale e un’efficiente interazione tra le fasi di donatore e di accettore che li rendono promettenti alternative ai blend di polimero e fullerene. Un nanoibrido composto da grafene ossido ridotto e molecole di trifenilammina legati covalentemente, è stato studiato come potenziale colorante per la titania in celle solari sensibilizzate a colorante, capace di migliorare la conducibilità e la stabilità del sistema. L’EPR ha mostrato che un efficiente trasferimento elettronico fotoindotto avviene tra l’ibrido e il semiconduttore, aprendo la strada all’applicazione di una nuova classe di coloranti. Infine, la fotoattività di un materiale supramolecolare, un gel composto da piccole molecole di donatore e accettore che autoassemblano, è stata studiata. In questo caso l’EPR ha permesso di verificare un efficiente trasporto di carica attraverso le strutture supramolecolari, suggerendo interessanti proprietà semiconduttive del materiale. I risultati di questa tesi dimostrano la rilevanza dell’EPR per l’indagine su aspetti funzionali e morfologici di materiali fotovoltaici e forniscono una caratterizzazione della fotofisica di nuovi sistemi che potrebbero essere ulteriormente esplorati per apportare progressi sostanziali nel campo del fotovoltaico organico e ibrido

Peptide-driven charge-transfer organogels built from synergetic hydrogen bonding and pyrene–naphthalenediimide donor–acceptor interactions

\u3cp\u3eThe peptide-driven formation of charge transfer (CT) supramolecular gels featuring both directional hydrogen-bonding and donor–acceptor (D-A) complexation is reported. Our design consists of the coassembly of two dipeptide–chromophore conjugates, namely diphenylalanine (FF) dipeptide conveniently functionalized at the N-terminus with either a pyrene (Py-1, donor) or naphthalene diimide (NDI-1, acceptor). UV/Vis spectroscopy confirmed the formation of CT complexes. FTIR and \u3csup\u3e1\u3c/sup\u3eH NMR spectroscopy studies underlined the pivotal role of hydrogen bonding in the gelation process, and electronic paramagnetic resonance (EPR) measurements unraveled the advantage of preorganized CT supramolecular architectures for charge transport over solutions containing non-coassembled D and A molecular systems.\u3c/p\u3

Probing photoinduced electron-transfer in graphene-dye hybrid materials for DSSC

We investigated the photophysical properties of a newly synthesized hybrid material composed of a triphenylamine dye covalently bound to reduced graphene oxide, potentially relevant as a stable photosensitizer in dye-sensitized solar cells. The photophysical characterization has been carried out by means of fluorescence quenching and fluorescence lifetime measurements, complemented by Electron Paramagnetic Resonance (EPR) spectroscopy, aimed at the detailed description of the photoinduced processes occurring in the hybrid and in the mixed hybrid/N-doped TiO2 material. The combined optical/magnetic study unequivocally demonstrates a fast quenching of the dye excited state in the isolated hybrid and an efficient electron transfer to N-doped titania nanopowders. In the latter case, a metastable radical cation on the dye moiety is photogenerated and the corresponding negative charge, an electron, is trapped in defect sites of the doped semiconductor oxide. The spin distribution in the stable radical has been determined by EPR spectroscopy and correlated with DFT calculations